Fix some undefined terms

[ektrah]

Here's my attempt at resolving a first portion of #704. It might be easier to review the individual commits than the whole pull request. Please let me know if I'm on the right track 😊

[steveraysteveray]

These changes all seem reasonable. I will wait until my QUDT colleagues have a chance to weigh in though.

Also, be aware that we are currently discussing the modeling of datatypes in SHACL, which raises a number of issues, so things may be changing. But (in my opinion), it's good to clean house with what we have anyway.

[steveraysteveray]

We finally got a chance to discuss this and we're fine with your suggestions except for changing the prefix qudt.type to qudt. This is because we generally use qudt: as a prefix for schema entities (classes, properties...) and other prefixes for the vocabulary/instances (e.g. unit:, quantitykind:, constant:). After some back and forth, we settled on qudt_type: as the preferred prefix name rather than qudt.type or just qudt. If you make that change, I think we're good.