make IsotonicRegression pickleable
import pickle
FIX: Thresholded Nearest Centroid fails with non-encoded y
Merge pull request #3746 from MechCoder/nearest_neighbor_manhattan

FIX: Thresholded Nearest Centroid fails with non-encoded y
Merge pull request #3669 from pprett/fix-isotonic-pickling

[MRG+1] Make IsotonicRegression pickleable
[MRG] Add quantile strategy to DummyRegressor. Fixes #3421.