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
WIP: some debug
WIP: some more debug
WIP: debug in progress...
WIP: debug (tests still don't pass...)
WIP: one more bug fixed
WIP: cleanup