Allows CART classification trees to produce pure leaves #303
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* Removing a check that prevents pure splits from being added. * Making the leaf node detection more robust. * Adding a test for pure leaf creation.
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Description
Removes a check that the left and right nodes have non-zero impurities. These values (on either or both sides) could be zero when the nodes are pure (i.e. have a single output label), and that should be allowed in the tree creation. Removing this check could cause trees to overfit more if built until pure, but that's what the CART algorithm says should happen, and it is expected in ensembles like Random Forests.
Motivation
The original check was invalid and prevents the model from making pure leaf nodes which only contain a single output value, potentially causing poor performance. Fixes #298.