sigma_error.f90 running, n= 100
 x 1 .. 100
 sum x^2 - (sum x)^2/n  sigma1 =  28.86607
 sum(x-mean)2/n  sigma2 =  28.86607
 x 10 .. 1000
 sum x^2 - (sum x)^2/n  sigma1 =  288.6607
 sum(x-mean)2/n  sigma2 =  288.6607
 x 100 .. 10000
 sum x^2 - (sum x)^2/n  sigma1 =  2886.606
 sum(x-mean)2/n  sigma2 =  2886.607
 x 1000 .. 100000
 sum x^2 - (sum x)^2/n  sigma1 =  28866.07
 sum(x-mean)2/n  sigma2 =  28866.07
 x 10000 .. 1000000
 sum x^2 - (sum x)^2/n  sigma1 =  288660.7
 sum(x-mean)2/n  sigma2 =  288660.7
 x 10001 .. 10100
 sum x^2 - (sum x)^2/n  sigma1 =  29.00190
 sum(x-mean)2/n  sigma2 =  28.86607