Add "Global" R-squared

[TaylorOshan]

Currently, results include a local R-squared that is based on weighted diagnostics and not directly comparable to the R-squared of a an OLS. Would be useful to have global R-squared implemented as:

R-squared = 1 - (RSS / TSS)

where

TSS = np.sum((results.y.reshape((815,1)) - np.mean(y))**2)
RSS = np.sum((results.y.reshape((815,1)) - results.predy.reshape((815,1)))**2)

[Ziqi-Li]

This is added in #18