More generic generic.stats - Allow 3H inputs

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes an issue raised in discussions with @vindelico
• 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?

• Relax indicator generic.STATS so it accepts non-daily inputs. I saw no particular reason for it to be limit to daily values, except from the metadata.
• Add variance to the supported op of to_agg_units.
• Add a out_units to select_resample_op so that one can use it with an op to_agg_units doesn't know, and explicitly pass the expected units.

Does this PR introduce a breaking change?

Not a big one, but all YAML indicators previous constructed from select_resample_op should add out_units: null in their parameters section to remove the new argument from the signature. Altough its presence is not problematic in itself.

Other information:

[huard]

Maybe add a quick test ?

[aulemahal]

Woups good call.

Because the initial issue of Marco was with 3H input, I tested with that. Turns out MissingBase wasn't able to comprehend frequencies containing multiples. I fixed that too.