Refactor and clean IsotonicRegression

Refactor and clean IsotonicRegression:

* Rename ``X`` to ``x`` to properly reflect that input is a vector, not matrix
* Refactor the ``y=isotonic_regression(...)`` logic into ``_build_y``
* Refactor the ``interp1d`` logic into ``_build_f``
* Store ``f=interp1d(...)`` so that it is not re-calculated on each call to ``transform``