Add Variance Reduction to Delta Method

[https://srome.github.io/CUPED-with-Multiple-Covariates-and-A-Simpler-Delta-Method-Calculation/](https://srome.github.io/CUPED-with-Multiple-Covariates-and-A-Simpler-Delta-Method-Calculation/). Add CUPED and hook up with CUPAC variance reduction techniques.
