Allows computation of mean, stdev, variance, covariance, Pearson correlation online. Getters for stdev, var, cov, cor cost floating point division ops. Getters for means cost a single branch to check for N == 0. This struct uses O(1) space.
PearsonCor.cor is alias this'd, so if this struct is used as a float, it will be converted to a simple correlation coefficient automatically.
Combine two PearsonCor's.