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``

Sorry about the last one, @agramfort.  Reset and sync'd.
The `order_inv` local variable is useless. I would just call:

```python
self._build_y(X, y, sample_weight)
```
to set `self.y_`.