Use the same logic for handling --mets/--mets-basename/--working-dir in processors and "ocrd workspace" CLI

[kba]

See #693 . This PR factors the resolving logic for the CLI options of ocrd workspace out to resolve_mets_arguments and reuses that in the processor wrapper, so they should follow the same logic sans the --mets-basename which is not part of the processor CLI spec (should it be?).

[kba]

I'd prefer if the exception / check could remain in place. Why don't you simply do the abspath thing to the processor decorator's --mets option as well?

(cannot reply directly to comments on outdated code apparently) Can you have another look at the code pls? I think I have cleaned up the logic properly now. The reason we cannot easily do abspath(--mets) is because --mets might be a proper URL not a filename.