Evaluation over training dataset (explanatory power)

[dinilu]

When fitting models, I generally like to compare predictive power (evaluation metrics over the testing datasets) against explanatory power (evaluation metrics over the training datasets). This is also an indicator of overfitting. I dont see any function in the package that allow to calculate explanatory power, just predictive power in the models objects. Maybe I am missing something. If not, I would like to see this option in the package.