WIP : Isotonic calibration

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

We have to decide how we average probabilities of the internal ensemble. Currently it uses the arithmetic mean but the geometric mean (average of logs) might be more appropriate in certain situations. We could also make this configurable.