Metadata: "Save datastack" should include metadata documents in the archive

[davemfish]

Saving a datastack refers to saving the input data for an model. There will not necessarily be metadata documents present, but if there are, we might consider including them in the datastack archive. We want to promote sharing metadata along with datasets.

We might also want to generate a metadata document for the archive itself.

[davemfish]

Actually, given that we have the ARGS_SPEC with lots of relevant metadata about model inputs, maybe we should generate metadata documents as part of the Save Datastack operation. That would also make it easier to discover & include in the archive.

It may also be useful to have an option to create metadata for an args dict without compressing & archiving the data.

Any pre-existing metadata should be preserved. geometamaker is designed to do that, but we will need to take care not overwrite descriptions or other human-modified attributes.