WIP : Isotonic calibration

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

I looked at this line, I think @mblondel might be right: we should average the plain probabilities. It's not like we are collecting evidence, rather we're asking experts. I'll ask some people. It might also be related to the measure: if it's Brier, average the probabilities, if it's logistic, the log probabilities.