COSMIT refactor random forests

Very little subclass-specific stuff left in base classes.

Also fix faulty import in forest tests.
COSMIT refactor forests, part 2

Decouple RandomTreesEmbedding from ForestRegressor.
FIX ^ is a symetric difference
Style fixes
[DOC] missing parameter description
MAINT Set random_state, modernize tests
TST max_features for more tree estimators
TST remove unused tests
ENH add missing pxd of utis.random
ENH Use file configuration
FIX signature
cosmetic: Changed all instances of nonlinear to non-linear
Merge pull request #2187 from FedericoV/non_negative_style

cosmetic: Changed all instances of nonlinear to non-linear
FIX faulty import in 20news docs
COSMIT sphinx fixes
Merge branch 'master' into ENH_docstrings_in_gallery
syntax fix