Take units from cf_xarray

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fulfills some goals of Phase out convert_calendar and some units processing #1010.
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• The units registry is copied from cf_xarray and extended with xclim's specific shortcuts (precipitation, discharge and the hydro context).
• The units formatting functions are simplified by using the work alreadty done in cf_xarray. We still can't get rid of pint2cfunits because of pint issue that makes it impossible to format dimensionless units are "" using its formatter tools.

Does this PR introduce a breaking change?

Yes. As seen in the tests changes, cf_xarray's cf formatter never adds the "^" sign for exponentiation. This does not affect the accepted units, only the output units attribute.

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[Zeitsperre]

Well done!