ENH bagging meta-estimator
ENH: move _partition_estimators to ensemble.base
TST: test_base in sklearn.ensemble
BUG: do not force base estimators to inherit from sklearn in AdaBoost
TST: fix random_state in test_weight_boosting
DOC: base_estimator_ + estimators_ in ensemble.base
COSMIT: tidy up forest code (inspired by bagging)
Merge branch 'bagging'

Fixes #2375.