Improve desciption of inverse_map and add option to direclty pass coordi...

See https://github.com/scikit-image/scikit-image/issues/1141
I don't think speed is an issue when transposing, unless you need it to be contiguous.  In which case, a transpose followed by another transpose *is* continuous.  The rest is solved by good documentation...

```python
In [1]: a = np.ones((1000, 1000))

In [8]: %timeit a * 0.1
100 loops, best of 3: 3.29 ms per loop

In [9]: %timeit a.T * 0.1
100 loops, best of 3: 3.45 ms per loop
```