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

See https://github.com/scikit-image/scikit-image/issues/1141
After `inverse_map = ProjectiveTransform(matrix=inverse_map)`, you must also run `coords = warp_coords(coord_map, output_shape)`, which your version does not, if I did not overlook sth. ;-)