Check for discharge unit compliance in streamflow statistical indicators

[huard]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes frequency_analysis indicator weirdly declared #1130
• 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?

• Check input data compliance with discharge units in streamflow indicators.
• Add discharge variable to variables.yml
• Add test confirming failure and informative error message if units do not comply.
• Deprecate generic streamflow indicators (fit, stats, freq_analysis)
• Create generic indicator realm with the indicators fit, stats, and return_level.

Does this PR introduce a breaking change?

Three land indicators for streamflow analysis have been deprecated.

Other information:

[RondeauG]

Looks good to me!

One remaining issue is that da is very nondescript and doesn't make is obvious that a discharge is required. If we followed conventions established for other functions in xclim, I'd expect q or something similar. However, I understand that changing it at this point would likely be a breaking change...

[huard]

It's because the stats indices are generic, and are meant to be applicable to any kind of variable.

Unless @aulemahal has magic trick up his sleeve ?

[huard]

This PR took an unexpected turn. It now includes a new generic realm to hold stats, fit and return_level (previously freq_analysis.

[RondeauG]

I'll let the others approve the more internal changes, but it looks good to me.

[coveralls]

Coverage increased (+0.03%) to 92.022% when pulling 542708f on fix-1130 into 503c506 on master.