FIX: Handle alpha channel in grey conversion

- Alpha channel is handled by compositing image on white background.
Alpha is normalized to its max value in the image, to be able to handle
different datatypes.
- Typo in ValueError

Follow-up on https://github.com/soft-matter/pims/pull/117
See https://github.com/matplotlib/matplotlib/issues/2473 for why (0, 0, 0, 0) should go to white.  I am in favor of leaving it as it is (and making sure the docs make it clear the built-in to_gray just drop alpha) for now and dealing with this when we have a user who really does have RGBA images being run through PIMS.  There is also the machinery in place for users to pass in their own conversion code so they can handle alpha exactly as they want.

In general, I think I want to err on the side of not shipping a feature over shipping a broken/confusing feature and err on the side of solving problems we actually have rather than imagined problems.