Add Natural Numbers Object#102
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This PR adds the property of having a natural number object, suggested in #94.
It uses the "parametrized" definition since this appears to be the correct one for non-cartesian closed categories.
Because of the useful lemma "countably distributive ===> NNO" I also added the property "countably distributive" (and its dual). This is in between "distributive" and "infinitary distributive", both of which were already present.
All three new properties have been decided for all categories in the database.
It is worth mentioning that Met does not have a NNO, but Met∞ does.
ℹ️ This PR has been created during recording a video about how to contribute to CatDat. Hence also the unusual account.