Adapt to current pint

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Pint 0.20 incompatible with Xclim unit registry processes #1211
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added
• The relevant author information has been added to AUTHORS.rst and .zenodo.json

What kind of change does this PR introduce?

• Changes how the UnitRegistry is declared, by copying the one from cf-xarray.
• I also copied the declaration of the % unit from cf-xarray
• This removed the need to import things that were refactored in pint0.20, so it fixes this.
• It also allowed to remove the string processing stuff from units2pint, this is now directly in the registry's preprocessors.
• I removed the force_ndarray_like=True option that cf-xarray uses because it made our tests fail. I haven't looked further, this is to be done in Phase out convert_calendar and some units processing #1010.

Does this PR introduce a breaking change?

It shouldn't!