Added quadratic mean (#7315)

* Added quadratic mean

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

* Added tests for quadratic mean

* Corrected quadratic mean

* Added comment to quadratic mean

* Corrected quadratic mean tests

* Replaced sqrt by pow

* Error fixed

* Extra whitespace removed

* Extra whitespace removed

* Removed extra white space

* Removed extra white space