Internal Decision Tree node.
param: prediction Prediction this node would make if it were a leaf node
param: impurity Impurity measure at this node (for training data)
param: gain Information gain value. Values less than 0 indicate missing values;
this quirk will be removed with future updates.
param: leftChild Left-hand child node
param: rightChild Right-hand child node
param: split Information about the test used to split to the left or right child.