Allow GROUP BY variables with parentheses or duplicates

[joka921]

With this change, grouped variables can be put in parentheses and still be used in the SELECT clause. For example, GROUP BY (?x) is now handled exactly like GROUP BY ?x. Additionally, we now deduplicate the grouped variables. For example, GROUP BY ?x ?x is now equivalent to GROUP BY ?x.

[sparql-conformance]

Conformance check passed ✅

No test result changes.

Details: https://qlever.cs.uni-freiburg.de/sparql-conformance-ui?cur=05f67f3cba50dced3499752b94b6b51541cf549b&prev=a97905ebaa4eff2005171706ee520449ce767f18

[sonarqubecloud]

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 SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.86%. Comparing base (a97905e) to head (05f67f3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1685   +/-   ##
=======================================
  Coverage   89.86%   89.86%           
=======================================
  Files         389      389           
  Lines       37189    37197    +8     
  Branches     4196     4198    +2     
=======================================
+ Hits        33419    33427    +8     
- Misses       2473     2475    +2     
+ Partials     1297     1295    -2     

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[hannahbast]

So many bug fixes for Christmas, thanks!