Fix use of offset units for higher dimensional units (e.g. gauge pressure #1066)

[dalito]

• Closes Absolute vs. Relative pressure #1066
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

The problem was limited to offset units of higher dimension like pressure (M L^-1 T^-2). The offset units included in pint are all temperature units with just one dimension and were thus not affected.

[jules-ch]

@dalito can you rebase & push to resolve conflict on changes

[jules-ch]

Great, merging this.
Thanks @dalito