workspace bagger: allow filtering file groups

[bertsky]

I know this is already on the agenda, but it should be more visible IMHO: ocrd zip bag could really use an option that allows filtering (negatively) or selecting (positively) those file groups which to keep in the bag.

Note: This may be more difficult than it seems. There are implicit dependencies between file groups, e.g. derived images in image file groups referenced (via AlternativeImage) from PAGE-XML file groups.

In my makefileization, I first zip bag everything and then workspace remove-group parts which I don't want as a stopgap measure. But this would be much easier in Python.

[kba]

Has been implemented in #1139