Post-pruning optimization to CART

[andrewdalpino]

The current implementation of CART (Classification and Regression Tree) only utilizes pre-pruning tactics to control overfitting. It has been shown that combining pre-pruning and post-pruning can achieve better regularization effects while speeding up inference. I propose a lightweight heuristic that will work with the impurity system already in place such that nodes with minimal impurity decrease can be pruned.

I am open to other suggestions.