make IsotonicRegression pickleable
import pickle
fix svm.fit in feature_stacker.py
Merge pull request #3669 from pprett/fix-isotonic-pickling

[MRG+1] Make IsotonicRegression pickleable
Merge pull request #3742 from abhishekkrthakur/master

FIX remove svm.fit in feature_stacker.py
FIX: clamping was not handled correctly in LabelPropagation.
Test: Added unit test for hard clamping in LabelPropagation and LabelSpreading.