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
why scale by `max_alpha`?  That will force the most opaque pixel to be fully opaque.