Added quadratic mean

[kvadrik]

Added a quadratic mean of the given numbers, sqrt ((n1^2+n2^2+...+nk^2)/k).

• 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 algorithms include a corresponding test class that validates their functionality.
• 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 79.42%. Comparing base (24c2bea) to head (721174e).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7315      +/-   ##
============================================
- Coverage     79.43%   79.42%   -0.01%     
- Complexity     7061     7062       +1     
============================================
  Files           788      788              
  Lines         23120    23124       +4     
  Branches       4545     4545              
============================================
+ Hits          18365    18366       +1     
  Misses         4022     4022              
- Partials        733      736       +3     

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