Make some member functions of `GroupBy` `const` or even `static`

[RobinTF]

This makes the static analysis happier and (in the case of the static methods) avoids the passing around of fully-fledged GroupBy objects.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.30%. Comparing base (14f638e) to head (a2b815a). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1510 +/- ## ========================================== - Coverage 94.30% 94.30% -0.01% ========================================== Files 350 350 Lines 25921 25918 -3 Branches 3485 3485 ========================================== - Hits 24446 24443 -3 Misses 1433 1433 Partials 42 42 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud