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
OK, well said. Closing then. Thanks for your work on this, Casper. We will revisit when the need arises.