Added Sample Variance & SD

The existing methods calculate the 'population' variance and standard
deviation. Added methods to calculate the 'sample' versions of these
quantities, which provide unbiased estimates of the population
parameters when a sample is used to estimate.

P.S. Looks like there were some end-of-line spaces that got truncated automatically by my editor, I'll happily amend to remove these if the commit log noise is an issue.