Tree refinement.
RF tests.
Support for 3-dim arrays
Tree refinement by tree merging.
Cache trees during RF scoring
Refined trees preserved in the model for further model merging.
Refined tree leaf contains a new histogram.
Used tree in python matrix view.
Avoid non necessary checking in Py API
Refined trees - two strategies - (1) append all refined trees into main RF
model, (2) merge refined trees and overwrite RF model.