WIP : Isotonic calibration

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

>
> I wonder whether we should move calibration_plot to the calibration
> module instead since this is not a metric per-se. Any opinion?
>
done.

I also added a narrative doc.

I'll address the other remarks later. My time is up for now