make sure re-quantifying dimension coordinates works

[keewis]

Now that we use call_on_dataset the call does not fail because of xarray, but because we pass a Unit object to parse_units. This PR fixes that and adds tests to make sure it stays that way.

I also tightened the restriction on dimension coordinates, as it was possible to change their units by re-quantifying. For #47 we might want to partially lift this restriction again.

• [x] Closes #105
• [x] Tests added
• [x] Passes pre-commit run --all-files
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

[keewis]

I'll merge this now and add a PR to close #47 this week-end