Make IsotonicRegression pickleable

fixes https://github.com/scikit-learn/scikit-learn/issues/3666

implements ``__getstate__`` and ``__setstate__`` by removing the interpolation function and rebuilding it.

cc @amueller @jnothman 
I'm not that bold  -- but good questions... ``self.__dict__ = state`` should do the same - I defer to a Python guru @GaelVaroquaux @ogrisel what do you think?