Major Description
RegressionMetrics currently computes explainedVariance using summary.variance(1) (variance of the residuals) where the Wikipedia definition uses the residual sum of squares math.pow(summary.normL2(1), 2). The two coincide only when the predictor is unbiased (e.g. an intercept term is included in a linear model), but this is not always the case. We should change to be consistent.
