BUG: fix sparsetools tests in old scipy
WIP: new Cython interface for decision trees
WIP: comments on the Cython interface
WIP: Criterion interface and base class
WIP: ClassificationCriterion (reset, update)
WIP: Gini criterion
WIP: entropy criterion
WIP: remove n_left and n_right attributes
WIP: MSE criterion
WIP: tree class
WIP: tree algorithm
WIP: add_node
WIP: node_value
WIP: node_value
WIP: predict + apply
WIP: Random Splitter
WIP: splitter
WIP: Best Splitter
WIP: sort features
WIP: first pass on tree.py