refactor: SubsetSum

[alxkm]

Class cleanup.
Readability improvements.
Added tests.

• 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 50.79%. Comparing base (14916e6) to head (fe5e948).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5432      +/-   ##
============================================
+ Coverage     50.76%   50.79%   +0.02%     
+ Complexity     3139     3138       -1     
============================================
  Files           523      523              
  Lines         15119    15109      -10     
  Branches       2880     2875       -5     
============================================
- Hits           7675     7674       -1     
+ Misses         7121     7114       -7     
+ Partials        323      321       -2     

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