Repository: fatiando/harmonica (python)
Issue: Coordinates decimal issue in filters
**Description of the problem:**
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When using the new grid transformations, I noticed that several (at least `derivative_upward` and `gaussian_lowpass`) functions are slightly changing the easting coordinates of the dataarrays. I was attempting to add the resulting grids as a new dataset variable with:
```python
magnetic_grid_padded['deriv_upward'] = hm.derivative_upward(magnetic_grid_padded)
```
Plotting the new grid shows:
![image](https://user-images.githubusercontent.com/81199856/225128479-bdca1ee7-5873-4d27-89d5-dd97f0726a7f.png)

Compared to the normal method of creating a new array shows an issue with the high and low easting values.
```python
deriv_upward = hm.derivative_upward(magnetic_grid_padded)
```
![image](https://user-images.githubusercontent.com/81199856/225128711-fc0a8e08-b976-4d45-af7e-89f90b86c4a4.png)

Numpy testing confirms that the easting values of the produced grid don't match the original past the 10th decimal place.

```python
np.testing.assert_almost_equal(
    deriv_upward.easting.values, 
    magnetic_grid_padded.easting.values, 
    decimal=11,
)
```
```
AssertionError: 
Arrays are not almost equal to 11 decimals

Mismatched elements: 391 / 576 (67.9%)
Max absolute difference: 1.16415322e-10
Max relative difference: 2.53196054e-16
 x: array([459783.31767976726, 459833.31767976726, 459883.31767976726,
       459933.31767976726, 459983.31767976726, 460033.31767976726,
       460083.31767976726, 460133.31767976726, 460183.31767976726,...
 y: array([459783.31767976715, 459833.31767976715, 459883.31767976715,
       459933.31767976715, 459983.31767976715, 460033.31767976715,
       460083.31767976715, 460133.31767976715, 460183.31767976715,...
```
The northing coordinates are exactly equal for this dataset, but northing and easting both fail the test on a dataset of min. 

I have a workaround, which is to round the coordinate values to the same number of decimal places before merging, which gives the desired results.

```python
magnetic_grid_padded = magnetic_grid_padded.assign_coords(
    {
        "easting":np.round(magnetic_grid_padded.easting.values),
        "northing":np.round(magnetic_grid_padded.northing.values)
    })

deriv_upward = hm.derivative_upward(magnetic_grid_padded)
deriv_upward = deriv_upward.assign_coords(
    {
        "easting":np.round(deriv_upward.easting.values),
        "northing":np.round(deriv_upward.northing.values)
    })

magnetic_grid_padded['deriv_upward'] = deriv_upward
```

I need to add the resulting data back to the original dataset since my grid has NaN's, which I've filled with -999 and need to mask the results with the original data extent. 

Any ideas what's causing this issue?

Otherwise, these new functions are great!

**System information**

* Operating system: Linux
* Python installation (Anaconda, system, ETS):  mambaforge
* Version of Python: 3.10.9
* Version of this package: 0.6.0.post1+g627fbfa.d20230310 
* If using conda, paste the output of `conda list` below:

<details>
<summary>Output of conda list</summary>
<pre>

# Name                    Version                   Build  Channel
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anyio                     3.6.2              pyhd8ed1ab_0    conda-forge
argon2-cffi               21.3.0             pyhd8ed1ab_0    conda-forge
argon2-cffi-bindings      21.2.0          py310h5764c6d_3    conda-forge
asttokens                 2.2.1              pyhd8ed1ab_0    conda-forge
attrs                     22.2.0             pyh71513ae_0    conda-forge
babel                     2.12.1             pyhd8ed1ab_1    conda-forge
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backports.functools_lru_cache 1.6.4              pyhd8ed1ab_0    conda-forge
beautifulsoup4            4.11.2             pyha770c72_0    conda-forge
bleach                    6.0.0              pyhd8ed1ab_0    conda-forge
bokeh                     2.4.3              pyhd8ed1ab_3    conda-forge
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h5netcdf                  1.1.0              pyhd8ed1ab_1    conda-forge
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jupyter_server_terminals  0.4.4              pyhd8ed1ab_1    conda-forge
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libpng                    1.6.39               h753d276_0    conda-forge
libsodium                 1.0.18               h36c2ea0_1    conda-forge
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libtiff                   4.5.0                hddfeb54_5    conda-forge
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libxml2                   2.10.3               h7463322_0    conda-forge
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nest-asyncio              1.5.6              pyhd8ed1ab_0    conda-forge
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prometheus_client         0.16.0             pyhd8ed1ab_0    conda-forge
prompt-toolkit            3.0.38             pyha770c72_0    conda-forge
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psutil                    5.9.4           py310h5764c6d_0    conda-forge
pthread-stubs             0.4               h36c2ea0_1001    conda-forge
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pycparser                 2.21               pyhd8ed1ab_0    conda-forge
pygments                  2.14.0             pyhd8ed1ab_0    conda-forge
pyopenssl                 23.0.0             pyhd8ed1ab_0    conda-forge
pyparsing                 3.0.9              pyhd8ed1ab_0    conda-forge
pyproj                    3.4.1           py310h15e2413_1    conda-forge
pyrsistent                0.19.3          py310h1fa729e_0    conda-forge
pyshp                     2.3.1              pyhd8ed1ab_0    conda-forge
pysocks                   1.7.1              pyha2e5f31_6    conda-forge
python                    3.10.9          he550d4f_0_cpython    conda-forge
python-dateutil           2.8.2              pyhd8ed1ab_0    conda-forge
python-fastjsonschema     2.16.3             pyhd8ed1ab_0    conda-forge
python-json-logger        2.0.7              pyhd8ed1ab_0    conda-forge
python_abi                3.10                    3_cp310    conda-forge
pytz                      2022.7.1           pyhd8ed1ab_0    conda-forge
pyyaml                    6.0             py310h5764c6d_5    conda-forge
pyzmq                     25.0.0          py310h059b190_0    conda-forge
readline                  8.1.2                h0f457ee_0    conda-forge
requests                  2.28.2             pyhd8ed1ab_0    conda-forge
rfc3339-validator         0.1.4              pyhd8ed1ab_0    conda-forge
rfc3986-validator         0.1.1              pyh9f0ad1d_0    conda-forge
scikit-learn              1.2.2           py310h209a8ca_0    conda-forge
scipy                     1.10.1          py310h8deb116_0    conda-forge
send2trash                1.8.0              pyhd8ed1ab_0    conda-forge
setuptools                67.6.0             pyhd8ed1ab_0    conda-forge
shapely                   2.0.1           py310h8b84c32_0    conda-forge
six                       1.16.0             pyh6c4a22f_0    conda-forge
sniffio                   1.3.0              pyhd8ed1ab_0    conda-forge
sortedcontainers          2.4.0              pyhd8ed1ab_0    conda-forge
soupsieve                 2.3.2.post1        pyhd8ed1ab_0    conda-forge
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stack_data                0.6.2              pyhd8ed1ab_0    conda-forge
tblib                     1.7.0              pyhd8ed1ab_0    conda-forge
terminado                 0.17.1             pyh41d4057_0    conda-forge
threadpoolctl             3.1.0              pyh8a188c0_0    conda-forge
tinycss2                  1.2.1              pyhd8ed1ab_0    conda-forge
tk                        8.6.12               h27826a3_0    conda-forge
tomli                     2.0.1              pyhd8ed1ab_0    conda-forge
toolz                     0.12.0             pyhd8ed1ab_0    conda-forge
tornado                   6.2             py310h5764c6d_1    conda-forge
traitlets                 5.9.0              pyhd8ed1ab_0    conda-forge
typing-extensions         4.4.0                hd8ed1ab_0    conda-forge
typing_extensions         4.4.0              pyha770c72_0    conda-forge
tzdata                    2022g                h191b570_0    conda-forge
unicodedata2              15.0.0          py310h5764c6d_0    conda-forge
urllib3                   1.26.14            pyhd8ed1ab_0    conda-forge
verde                     1.7.0              pyhd8ed1ab_0    conda-forge
wcwidth                   0.2.6              pyhd8ed1ab_0    conda-forge
webencodings              0.5.1                      py_1    conda-forge
websocket-client          1.5.1              pyhd8ed1ab_0    conda-forge
wheel                     0.38.4             pyhd8ed1ab_0    conda-forge
xarray                    2023.2.0           pyhd8ed1ab_0    conda-forge
xorg-libxau               1.0.9                h7f98852_0    conda-forge
xorg-libxdmcp             1.1.3                h7f98852_0    conda-forge
xrft                      1.0.1                    pypi_0    pypi
xz                        5.2.6                h166bdaf_0    conda-forge
y-py                      0.5.9           py310h4426083_0    conda-forge
yaml                      0.2.5                h7f98852_2    conda-forge
ypy-websocket             0.8.2              pyhd8ed1ab_0    conda-forge
zeromq                    4.3.4                h9c3ff4c_1    conda-forge
zict                      2.2.0              pyhd8ed1ab_0    conda-forge
zipp                      3.15.0             pyhd8ed1ab_0    conda-forge
zstd                      1.5.2                h3eb15da_6    conda-forge

</pre>
</details>

Pull request description: Fix coordinate rounding errors in FFT
Here is a quick fix for the rounding error of the coordinate values during the FFTs. I've simply copied the original grid coordinates back to the grid after filtering.

This now allows users to add the filtered results as a new variable to the dataset/dataarray:
```python
magnetic_grid_padded["upward_continued"] = hm.upward_continuation(magnetic_grid_padded, height_displacement=500)
```
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**Relevant issues/PRs:**
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Fixes #395 

Patch:
diff --git a/harmonica/filters/_utils.py b/harmonica/filters/_utils.py
index bd2b7c4..0325386 100644
--- a/harmonica/filters/_utils.py
+++ b/harmonica/filters/_utils.py
@@ -61,4 +61,10 @@ def apply_filter(grid, fft_filter, **kwargs):
     filtered_fft_grid = fft_grid * da_filter
     # Compute inverse FFT
     filtered_grid = ifft(filtered_fft_grid).real
+
+    # use original coordinates on the filtered grid
+    filtered_grid = filtered_grid.assign_coords(
+        {dims[1]: grid[dims[1]].values, dims[0]: grid[dims[0]].values}
+    )
+
     return filtered_grid


Test patch:
diff --git a/harmonica/tests/test_filters.py b/harmonica/tests/test_filters.py
index 42fe0e7..035af9d 100644
--- a/harmonica/tests/test_filters.py
+++ b/harmonica/tests/test_filters.py
@@ -220,6 +220,18 @@ def test_apply_filter_grid_with_nans(invalid_grid_with_nans):
         apply_filter(invalid_grid_with_nans, dummy_filter)
 
 
+def test_coordinate_rounding_fix(sample_grid):
+    """
+    Check that the transformed grid has the same coordinates as the input grid.
+    """
+    # Apply the dummy filter
+    filtered_grid = apply_filter(sample_grid, dummy_filter)
+
+    # Compare coordinates of original grid with coordinates of filtered grid
+    npt.assert_array_equal(filtered_grid.easting.values, sample_grid.easting.values)
+    npt.assert_array_equal(filtered_grid.northing.values, sample_grid.northing.values)
+
+
 # -----------------------------
 # Test upward derivative filter
 # -----------------------------