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

This can be done with `bincount`

```python
    bin_sums = np.bincount(binids, weights=y_score, minlength=bins)
    bin_true = np.bincount(binids, weights=y_true, minlength=bins)
    bin_total = np.bincount(binids, minlength=bins)
    nonzero = bin_total != 0
    prob_true = (bin_true / bin_total)[nonzero]
    prob_pred = (bin_sums / bin_total)[nonzero]
```