FIX: weight impurity improvement by the size of the node
DOC: better doc for impurity_improvement
ENH: declare weighted_n_samples
ENH: rename Node attributes to avoid confusion
ENH: rename normalizer -> weighted_n_samples
ENH: renamed things too fast...
ENH: better API for passing weighted_n_samples
ENH: remove unused variables
Merge pull request #3056 from glouppe/tree-bestfirst

[MRG] Trees: Fix inconsistency in impurity_improvement