Add q (streamflow) to variables.yml (#1996)

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### 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)
- [x] CHANGELOG.rst has been updated (with summary of main changes)
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added

### 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.

CHANGELOG.rst

@@ -4,7 +4,7 @@ Changelog
 
 v0.54.0 (unreleased)
 --------------------
-Contributors to this version: Trevor James Smith (:user:`Zeitsperre`), Pascal Bourgault (:user:`aulemahal`).
+Contributors to this version: Trevor James Smith (:user:`Zeitsperre`), Pascal Bourgault (:user:`aulemahal`), Éric Dupuis (:user:`coxipi`).
 
 Breaking changes
 ^^^^^^^^^^^^^^^^
@@ -20,6 +20,7 @@ Bug fixes
 Internal changes
 ^^^^^^^^^^^^^^^^
 * Changed french translations with word "pluvieux" to "avec précipitations". (:issue:`1960`, :pull:`1994`).
+* `streamflow` entry replaced with `q` in ``variables.yml``.  (:issue:`1912`, :pull:`1996`)
 * In order to address 403 (forbidden) request errors when retrieving data from GitHub via ReadTheDocs, the ``nimbus`` class has been modified to use an overloaded `fetch` method that appends a User-Agent header to the request. (:pull:`2001`).
 * Addressed a very rare race condition that can happen if `pytest` is tearing down the test environment when running across multiple workers. (:pull:`1863`).
 

xclim/data/variables.yml

@@ -237,7 +237,7 @@ variables:
     standard_name: lwe_thickness_of_snow_amount
     data_flags:
       - negative_accumulation_values:
-  streamflow:
+  q:
     canonical_units: m3 s-1
     cell_methods: "time: mean"
     description: The amount of water, in all phases, flowing in the river channel and flood plain.