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

See https://github.com/scikit-image/scikit-image/issues/1141
It's just that if the input matrix is 3-D and the output 1-D of size 3, there is a conflict with the homogenous transformation matrix... and there is no sane way to tell what the user wants. However, I see this as a really, really rare case...