FIX: release reference to X in _tree.splitter
Merge pull request #2363 from glouppe/tree-memory

FIX: release reference to X in _tree.splitter
DOC: fix metrics documentation format

Fixes #2306.
FIX+TST non-consecutive or duplicate vocabulary indices

* added extra checks for custom vocab in CountVectorizer
* added test for custom vocab to check fauly vocabs

Fixes #2357.
Better docstring for KMeans.predict.
Move estimate_bandwidth test to its own function.
Add predict method to MeanShift.
Add predict method to AffinityPropagation.
Add missing docstrings to test functions.
Add what's new item.
Remove warning in AffinityPropagation.
Fix grid search test.
More user-friendly error message.
COSMIT: change variable name.
DOC adding description for handling of the auto parameter in neighbors
Added cosine distance metric for sparse matrices

- Added 'cosine_distances' function to sklearn.metrics.pairwise.
- Added 'cosine' as metric in 'pairwise_distances' function.
- Corrected doc string of same function, because all metrics based on
the 'manhattan_distances' function (i.e. 'cityblock', 'l1', and
'manhattan') do currently NOT support sparse matrices.
- Added corresponding corresponding unit test.
added missing assert in unit test
Fixed doc string; compute cosine distance without copying matrix.
DOC make DictVectorizer docstring refer to FeatureHasher

... instead of to itself.
FIX remove warnings from univariate FS

These warnings are issued practically always when using
frequency-valued or boolean data.

Switched to a stable sort to get reproducible results.