Add --demo CLI option

[mccalluc]

• extract fake csv creation from Proposed demo dataset / output notebook / output CSV / output text #48: Once this is merged to main, I would plan to merge main to that branch, and remove the code from the notebook that is now covered here.
• Fix Implement --demo option #7 (though we don't do anything with the data yet)
• enable doctests
• Consolidate arg parsing and demo dataset creation in one file
• Related to Move get_args out of dataset_ui #51, but don't think it really fixes it.

There are a few pragma: no cover notes scattered over this where we are missing test coverage. If you want to hold this to a higher standard, I wouldn't object, but I think this is sufficient for now.

[ekraffmiller]

Issue created to add help text about demo data #75