Repository: ouranosinc/xclim
Issue: run_length functions with 'coord' parameter give error with cftime time index

* xclim version:0.17
* Python version:3.7
* Operating System: Linux

### Description
<!--Describe what you were trying to get done.
Tell us what happened, what went wrong, and what you expected to happen.-->
Running ``last_spring_frost`` or ``growing_season_end`` indices fail when data has time as cftime index
The problem seems to be in the run_length operations used with ``coord`` parameter being defined as ``'dayofyear'``.     I'm not sure what is happening exactly but has something do so with auto rechunking of dask arrays that does accept the fact the chunk dtype is an object?  

Note:
If I manually change the the call of ``last_spring_frost`` using ``coord=False`` things run ok.


### What I Did
Simple example here
```
import xarray as xr
from xclim import atmos, subset

url = 'https://pavics.ouranos.ca/twitcher/ows/proxy/thredds/dodsC/datasets/gridded_obs/nrcan.ncml'
ds= subset.subset_bbox(xr.open_dataset(url, chunks=dict(time=31)),
                        lat_bnds=[45, 50], lon_bnds=[-75, -70], start_date='1971', end_date='1975')
ds_cf = subset.subset_bbox(xr.open_dataset(url, chunks=dict(time=31), use_cftime=True),
                        lat_bnds=[45, 50], lon_bnds=[-75, -70], start_date='1971', end_date='1975')
## ok with datetime64
out = atmos.last_spring_frost(tas=ds.tasmin, window=1)
out.isel(time=0).plot()
## cftime timestamps fail
out_cf = atmos.last_spring_frost(tas=ds_cf.tasmin, window=1)
```

### What I Received
<!--Paste the output or the stack trace of the problem you experienced here.-->
```
Traceback (most recent call last):
  File "/home/logan/Downloads/pycharm-community-2020.1.1/plugins/python-ce/helpers/pydev/pydevd.py", line 1438, in _exec
    pydev_imports.execfile(file, globals, locals)  # execute the script
  File "/home/logan/Downloads/pycharm-community-2020.1.1/plugins/python-ce/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\\n", file, 'exec'), glob, loc)
  File "/home/logan/.config/JetBrains/PyCharmCE2020.1/scratches/scratch.py", line 13, in <module>
    out = atmos.last_spring_frost(tas=ds_cf.tasmin, window=1)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xclim/core/indicator.py", line 184, in __call__
    out = self.compute(**das, **ba.kwargs)
  File "<boltons.funcutils.FunctionBuilder-55>", line 2, in last_spring_frost
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xclim/core/units.py", line 361, in wrapper
    out = func(*args, **kwargs)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xclim/indices/_threshold.py", line 494, in last_spring_frost
    return cond.resample(time=freq).map(
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/resample.py", line 222, in map
    combined = super().map(func, shortcut=shortcut, args=args, **kwargs)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/groupby.py", line 811, in map
    return self._combine(applied, shortcut=shortcut)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/groupby.py", line 835, in _combine
    combined = concat(applied, dim)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/concat.py", line 135, in concat
    return f(objs, dim, data_vars, coords, compat, positions, fill_value, join)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/concat.py", line 447, in _dataarray_concat
    ds = _dataset_concat(
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/concat.py", line 396, in _dataset_concat
    combined = concat_vars(vars, dim, positions)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/variable.py", line 2384, in concat
    return Variable.concat(variables, dim, positions, shortcut)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/variable.py", line 1652, in concat
    data = duck_array_ops.concatenate(arrays, axis=axis)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/duck_array_ops.py", line 266, in concatenate
    return _concatenate(as_shared_dtype(arrays), axis=axis)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/xarray/core/duck_array_ops.py", line 47, in f
    return wrapped(*args, **kwargs)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/dask/array/core.py", line 3468, in concatenate
    seq = [asarray(a) for a in seq]
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/dask/array/core.py", line 3468, in <listcomp>
    seq = [asarray(a) for a in seq]
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/dask/array/core.py", line 3740, in asarray
    return from_array(a, getitem=getter_inline, **kwargs)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/dask/array/core.py", line 2767, in from_array
    chunks = normalize_chunks(
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/dask/array/core.py", line 2468, in normalize_chunks
    chunks = auto_chunks(chunks, shape, limit, dtype, previous_chunks)
  File "/home/logan/anaconda3/envs/xclim/lib/python3.8/site-packages/dask/array/core.py", line 2551, in auto_chunks
    raise NotImplementedError(
NotImplementedError: Can not use auto rechunking with object dtype. We are unable to estimate the size in bytes of object data
```


Changed files:
xclim/indices/run_length.py

diff --git a/xclim/indices/run_length.py b/xclim/indices/run_length.py
index 20c6504f..85dfc193 100644
--- a/xclim/indices/run_length.py
+++ b/xclim/indices/run_length.py
@@ -250,7 +250,7 @@ def first_run(
 
         out = lazy_indexing(crd, out)
 
-    if dim in out:
+    if dim in out.coords:
         out = out.drop_vars(dim)
 
     return out