`import skimage` in submodules seems to cause issues with Sphinx autodocs. (Maybe some sort of circular import issue.) Note the `ImportErrors` fixed by this commit don't actually cause sphinx build errors; Sphinx seems to capture the errors, but it's annoyingly noisy, nonetheless.
Remove "greyscale" prefix on functions in ``morphology.grey``. To avoid confusion (and name clashes), ``greyscale_open`` and ``greyscale_close`` were renamed ``opening`` and ``closing`` (instead of ``open`` and ``close``). As a bonus, these names match scipy.ndimage.