Support for temperature differences in CF attributes

[huard]

Set units_metadata attribute to "temperature: difference" for arrays representing a difference between two temperatures.

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Use units_metadata where appropriate. #1822
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.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?

• Add units_metadata attribute set to "temperature: difference" whenever values represent a difference between two temperatures (degree days, diurnal amplitude, bias, etc).
• Adjust the units handling functions to recognize this attribute.
• Modify indices that set units to delta_degC, which is not CF compliant, by the original input DataArray units and the units_metadata attribute.
• Create new function pint2cfattrs to convert delta units into a dictionary of attributes, including units_metadata.

Does this PR introduce a breaking change?

Yes, units returned by some indices will change.

Other information:

I suspect I missed some instances where fixes are needed.

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[huard]

@juliettelavoie @aulemahal Ce PR va possiblement affecter des fonctions que vous avez créées.

[juliettelavoie]

lgtm!

[coveralls]

coverage: 89.356% (-0.1%) from 89.462%
when pulling 212725a on fix-1822
into 0a19979 on main.

[aulemahal]

Overall this looks good!

However, should we ensure that all indicators that give temperatures have the proper units_metadata? For example :

xc.atmos.daily_temperature_range(ds=ds)
# Attributes:
#    units:           K
#    units_metadata:  temperature: unknown

And while testing this I realized that:

xc.indices.daily_temperature_range(tasmin=tn, tasmax=tx)
#Attributes:
#    units:    delta_degC

Which isn't great as it's not CF. This problem predates your PR, but it would be nice to apply your changes to this indice!