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
There must be some normalization of the alpha channel, for which you have to know the max value (in most cases either 1.0 or 255). For int values, it depends bitdepth. I couldn't think of a way to obtain the bitdepth of the alpha channel correctly. So I chose the next best option, assuming that there are pixels with full opacity.