Improve the GMM PDF example.
FIX #1622: OPTIM: remove obsolete random_state instance in the Tree class

The random_state instance is now directly handled by the Splitter class,
there is no need to store it as an attribute of the cython Tree class
anymore.

This significantly speeds up the time of unpickling the trees by not having
to reconstruct pickled numpy.random.RandomState instances.
BUG: Compare strings for equality, not identity.

Strings in Python are usually interned (https://en.wikipedia.org/wiki/String_interning) but not always, so comparing identity can fail.
Merge pull request #2558 from Jorge-C/patch-1

BUG: Compare strings for equality, not identity.
Merge branch 'pr/2556'
Better title
COSMIT micro-optimize norm computation in NMF

... for clarity, although it should also be slightly faster, and
corrected a docstring while I was there.
DOC fix cross-decomposition docstrings
ENH: Make forest prediction code more robust to mutations of the estimators list

This can be useful to quickly study the impact of the size of the forest by
subsampling from it for instance.