WIP : Isotonic calibration

calibration module with platt and Isotonic calibration.

A few issues :

- [ ]  nice calibration plots
- [ ]  add tests for metrics
- [ ]  add narrative doc

This is precisely what we call `base_estimator` in the scikit-learn project:

```
git grep "base_estimator="
```

to see other meta estimator in scikit-learn. But arguably there are other meta estimator such as `RFE` and `GridSearchCV` that do not follow that convention.