DecisionTree optimization: eliminated filters + small changes

DecisionTree.scala
* Eliminated filters, replaced by building tree on the fly and filtering top-down.
** Aggregation over examples now skips examples which do not reach the current level.
* Only calculate unorderedFeatures once (in findSplitsBins)

Node: Renamed predictIfLeaf to predict

Bin, Split: Updated doc

Author: jkbradley