bug fix for issue #3526

Some of the pairwise distances do not support the additional `squared` parameter. I suggest using `sqeuclidian` and such whenever this is required.
bug fix for t-SNE (issue #3526) with new inputs

New fix following the discussion on the previous pull request. Thanks to @jnothman,  @mblondel and others ..
Remove trailing '\' and add test
Describe default metric
ENH: Patches Nearest Centroid for metric=manhattan for sparse and dense data
FIX: Wrap csc_row_median around the _get_median imputer function
MAINT: Move _get_median into sparsefuncs to avoid circular imports
DOC: Explain why the centroid of the manhattan metric is the median
Renamed csc_row_median to csc_median_axis_0
Warning for non-euclidean and non-manhattan metrics
Sparse matrix conversion depending on the type of metric

If metric is "manhattan", then store it as csc since calculating
the median is easier.
Update what's new.rst
Merge pull request #3772 from MechCoder/manhattan_metric

[MRG+2] ENH: Patches Nearest Centroid for metric=manhattan for sparse and dense data
MAINT remove deprecated oob_score_
Merge pull request #3786 from AlexanderFabisch/tsne_fix

Fix t-SNE with "non-squarable" metric
adding example for memory wrapper
Merge pull request #3795 from dsullivan7/svm_doc

[MRG+1] adding example for memory wrapper for load_svm_light
MAINT remove deprecated loss in gradient boosting
DOC add missing public function into the references