code: public static double xbarStd(TransposeDataCollection sampleDataCollection, int populationM, double Nbar) {
          return Math.sqrt(xbarVariance(sampleDataCollection, populationM, Nbar));
      }
docs: Calculates Standard Deviation for Xbar
  
  @param sampleDataCollection
  @param populationM
  @param Nbar
  @return