Add `q` (streamflow) to variables.yml

[coxipi]

Pull Request Checklist:

• [x] 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)
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What kind of change does this PR introduce?

• Add q as a proper variable in variables.yml

Does this PR introduce a breaking change?

No

Other information:

Discussing with @RondeauG , we were deliberating between qstr, qstrm, strf. I haven't seen any of these variables in many places in litterature, there doesn't seem to be a standard abbreviation apart from Q or q (flow or specific flow ), so I'm enclined to change strf if it doesn't feel right.

[aulemahal]

But if q is the most common standard, why would we not use that ?

[coxipi]

But if q is the most common standard, why would we not use that ?

I just thought that the variable associated with the streamflow should have its standard naming declared with units in variables.yml. Then, if we define q as a streamflow in variables.yml, I'm not sure it's a good idea to use it as a variable for quantiles, no?

[aulemahal]

Most references to q as "quantile" I see in xclim are in sdba, where I would assume context is enough to make the difference ? There are no indicators that refer to quantiles as q, only xc.indices.stats.parametric_quantile does it and I would argue again that context would be enough here.

I think my point is that such a breaking change doesn't seem justified if it is not based on convention.

[coxipi]

Ok, I've just replaced the streamflow entry in variables.yml with q then.