Returns the R-Squared for the nonparametric regression.
Notes
For more details see p.45 in [2] The R-Squared is calculated by:
R^{2}=\frac{\left[\sum_{i=1}^{n} (Y_{i}-\bar{y})(\hat{Y_{i}}-\bar{y}\right]^{2}}{\sum_{i=1}^{n} (Y_{i}-\bar{y})^{2}\sum_{i=1}^{n}(\hat{Y_{i}}-\bar{y})^{2}},
where \hat{Y_{i}} is the mean calculated in fit at the exog points.