Request for ongoing code review

Especially re https://github.com/valueflows/vocabulator/blob/master/vocab/models.py

As @fosterlynn mentioned, I am working with a relational data model. I can do a little inheritance, but it's also a little ugly.

I am doing this to get something deployed that is usable, and also because I want to support relational models as well as RDF-based models as well as plain old json and CSVs.

You can see the current output formats here: https://github.com/valueflows/vocabulator/blob/master/vocab/views.py#L20

More can be added, or if any of those are not needed, they can be removed.

This will all change over time. The current models are not fully implemented yet, but see next issue...