Add sampleStdev and sampleVariance methods.

[johnnaegle]

The stdev and variance functions, as exposed, continue to be population functions.

When the sample sizes are large enough, the values for sampleStdev and sampleVariance are very close. However, I was using a sample size of 6 and was seeing a large difference. Ultimately, I tracked this down to sample vs population standard deviations (and variances). This package uses stdev as a population functions where software such as postgres and applications such as google sheets use sample functions.

• Update the README with new functions
• Tests for sampleStdev and sampleVariance
• Expose 4 new functions: sampleStdev, populationStdev, sampleVariance, and populationVariance
• Alias stdev to populationStdev for compatibility
• Alias variance to populationVariance for compatibility
• Add test for populationStdev of only one thing

[johnnaegle]

I think I'd flip the names around:

stdevS, stdevP, varianceP, varianceS instead of the longer names sampleStdev, populationStdev, populationVariance, sampleVariance.

Thoughts?

[brycebaril]

I think I prefer the longer names, just for clarity. I've been thinking about other ways to expose this that might also make sense but I haven't come up with anything I like better yet.

[brycebaril]

Published as v2.2.0 Thanks!