Calculates the standard deviation of the elements in an array as root of
the variance = [sum( x(i) - mean)^2 )] / (n-1), where x(i) is the ith
element, m the mean, n the number of elements
Compares two columns by their name, returns -1 if f1 comes alphabetically
before f2, 0 if the two are equal and +1 if f2 comes alphabetically
before f1.
Compares two columns by their name, returns -1 if this column comes
alphabetically before f2, 0 if the two are equal and +1 if f2 comes
alphabetically before this column.