refactor: cleanup EulersFunction

[alxkm]

Javadocs improvements.
Tests improvements.

• 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 47.38%. Comparing base (f3851e3) to head (306708b).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5388      +/-   ##
============================================
- Coverage     47.39%   47.38%   -0.02%     
  Complexity     2923     2923              
============================================
  Files           524      524              
  Lines         15317    15317              
  Branches       2907     2907              
============================================
- Hits           7260     7258       -2     
- Misses         7757     7758       +1     
- Partials        300      301       +1     

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