Snow and radiation conversion, indicator translation fixes

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• 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

What kind of change does this PR introduce?

• Adds two new indices and indicators (snow_depth_from amount and snow_amount_from_depth) for converting between snow amount and snow depth (with the aid of snow density).
• Adds two new indices and indicators (longwave_radiation_upwards_from_net_downwards and shortwave_radiation_upwards_from_net_downwards) for determining rlus from rls and rlds, as well as rsus from rss and rsds, respectively.
• Adds the [power] unit definition for radiation-based variables.
• Fixed a bug that was affecting the validity checking of some indicator translations in test_xclim_translations.
  • Added missing translations for kbdi, df, ffdi and fixed broken translations for cffwis and jetstream_metric_woolings.
• Some minor adjustments in docstrings

Does this PR introduce a breaking change?

No.

Other information:

See: Ouranosinc/miranda#106

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[aulemahal]

While looking up the CF standard for rlus and family, I realized we were not using the same vocabulary... I think we should go for CF:
"surface downwelling shortwave flux" instead of "surface downwards solar radiation", and so on.

"Downwards" sound like a direction, while "downwelling" also has a meaning of source.

For snow conversion, I don't think these are "approximations". I would rename:

snow_amount_approximation -> snow_amount
snow_depth approximation -> snow_depth
snow_depth -> snow_depth_mean

(sry)