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