a3.r2: Cross-Validated R^2
In A3: Accurate, Adaptable, and Accessible Error Metrics for Predictive Models

Applies cross validation to obtain the cross-validated R^2 for a model: the fraction of the squared error explained by the model compared to the null model (which is defined as the average response). A pseudo R^2 is implemented for classification.

1	a3.r2(y, x, simulate.fn, cv.folds)

`y`	a vector or responses.
`x`	a matrix of features.
`simulate.fn`	a function object that creates a model and predicts y.
`cv.folds`	the cross-validation folds.