Add Junit tests for AffineCipher.java, improve documentation

[Hardvan]

• 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 54.74%. Comparing base (f34fe4d) to head (710c39b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5598      +/-   ##
============================================
+ Coverage     54.56%   54.74%   +0.18%     
- Complexity     3413     3426      +13     
============================================
  Files           535      536       +1     
  Lines         15383    15405      +22     
  Branches       2935     2937       +2     
============================================
+ Hits           8393     8433      +40     
+ Misses         6637     6618      -19     
- Partials        353      354       +1     

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