Have relative and absolute temperature units?

In #11204, we had to work around temperature units because K and C, F do not have the same origin. In principle, temperature *differences* are much easier to deal with and it might be worth having those be treated differently. Or have an offset unit class based on the functional units.

Start by reading issue #11204 and the existing temperature-unit behavior it discusses. Define whether relative temperature differences need distinct units or whether an offset-unit abstraction is appropriate, then identify the affected unit tests and document the expected conversions before implementation.

Written by the indexing model from the issue text.