Enh: Add Celsius

[bzah]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Add "Celsius" to pint registry #1067
• 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
• bumpversion patch has been called on this branch
• The relevant author information has been added to .zenodo.json

What kind of change does this PR introduce?

Add "Celsius" as a alias for "celsius" in pint registry.

Does this PR introduce a breaking change?

No

Other information:

I did the bump manually not using bumpversion but it should be fine.

[bzah]

I just saw that cf-xarray has added "degrees_Celsius" as another alias. It would not cost much to add it as well.

[coveralls]

Coverage increased (+0.002%) to 91.605% when pulling a3eb7b1 on enh/Celsius_unit into 4c8507f on master.