Add optional dataset category and frequency to align with aleph

[simonwoerpel]

This PR adds two optional properties to a dataset, frequency and category that are found in the aleph dataset model. (https://github.com/alephdata/aleph/blob/main/aleph/model/collection.py#L20)

It extends the nomenklatura.dataset.util.type_check function with an optional literal parameter to test for the membership of the value in this literal.

Additionally, the dataset tests are updated accordingly.

Hope that's a useful contribution, I personally definetly have the use case 🙃

[pudo]

Very cool, thanks so much for pulling this together with tests and everything! One thing that I'm a bit torn about is if it makes sense to replicate the enumerated values for categories in parallel to those in Aleph. The category scheme in aleph is broken to begin with (category error: leak vs. company registry for example), so I wonder if we shouldn't just use it as a string field in Nomenklatura which sometimes may happen to contain values aleph understands when fed via the API?

[simonwoerpel]

yes, agree for categories. if someone wants to integrate with aleph, just make sure the values match...
for frequencies: i suggest using https://github.com/Sonic0/cron-converter for cron-like string parsing & validation, what about this? this is a port of a similar JS library so the values could easily adapted to render on nice websites...