refactor: cleanup PigeonholeSort

[alxkm]

This PR adds missed test.

Improve code structure, this should add more readability.

Replaced ArrayList<Integer>[] pigeonHole = new ArrayList[numOfPigeonholes]; with List<List<Integer>> pigeonHoles = new ArrayList<>(maxElement + 1);.
Because such constructions in Java is generally not considered good practice.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.42%. Comparing base (76a450f) to head (7d3fc42).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5298      +/-   ##
============================================
+ Coverage     41.24%   41.42%   +0.18%     
- Complexity     2539     2552      +13     
============================================
  Files           520      520              
  Lines         15449    15447       -2     
  Branches       2953     2950       -3     
============================================
+ Hits           6372     6399      +27     
+ Misses         8782     8754      -28     
+ Partials        295      294       -1     

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