ocrd validate: accept --mets-basename, bashlib: use that

[kba]

When calling a bashlib processor like ocrd-olena-binarize with a workspace backed by a METS not called mets.xml, this leads to an error because we call ocrd validate tasks at the end of the bashlb argument parsing and it only specifies the --workspace (directory) so fails to instantiate the workspace, or even uses the wrong file.

I would like to consolidate the options to a single --mets option like we did for the processors and ocrd workspace option, but that will probably best happen in conjunction with the METS server change.

[bertsky]

Good catch!

We should probably also add some basename tests with the dummy processor (like test_mets_basename and test_ocrd_dummy.TestDummyProcessor combined).

Also, we have next to no tests for bashlib processing yet.

BTW, before ocrd-olena-binarize (and the rest of them) can actually handle the non-default basename case correctly, we would still have to rewrite all the ocrd workspace calls to something like ocrd workspace -m $(basename ${ocrd__argv[mets_file]}).

And so to facilitate that, in turn, we should probably add something like ocrd__workspace function to lib.bash (which would work both before and after the usual cd call)...

[kba]

Merged with the basename typo fix, but amended, so GH thinks there's unmerged changes here.