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

See https://github.com/scikit-image/scikit-image/issues/1141
I see. Well, I think Descartes beat those two to the punch. =P

More seriously, since Matplotlib uses XY as I defined it above, and we probably want compatibility with MPL in a bunch of places, I think it is useful to write CR/(col, row) for this case. We might also want to add a page to the documentation defining this. I'll raise an issue now for future reference.