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
typo: "requiered" -> "required"
Merge pull request #3057 from ajschumacher/patch-4

typo: "requiered" -> "required"
DOC no input validation in constructors
ENH: Add dynamic maximum trial determination to RANSACRegressor
DOC: typo: 1e-2 is 0.01 not 100
DOC remove import scikits.learn from tutorial

I think this is mostly a distraction here;
probably better for readers to take it out.
FIX: Add allow_nans option to check_arrays

Grid search and cross validation should not panic when seeing NaNs
in the input arrays, because that breaks Imputer.
Fixes #2774 and #3044.
ENH: remove unused variables
Merge pull request #3056 from glouppe/tree-bestfirst

[MRG] Trees: Fix inconsistency in impurity_improvement