Minor refactoring changes to `GroupBy` and related classes

[RobinTF]

These include improvements like

• Adding const qualifiers to functions
• Extracting subfunctions that will be reused in the future.
• Increasing readability by dropping redundant namespace qualifiers

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.85714% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.55%. Comparing base (5140166) to head (4083b11). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/engine/GroupByHashMapOptimization.h	75.00%	1 Missing and 1 partial :warning:
src/engine/Result.cpp	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1484 +/- ## ========================================== - Coverage 89.57% 89.55% -0.02% ========================================== Files 347 347 Lines 25620 25617 -3 Branches 3443 3445 +2 ========================================== - Hits 22949 22942 -7 - Misses 1483 1485 +2 - Partials 1188 1190 +2 ```

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[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