Integrate new version of the model that jointly models the competing risk incidences

[ogrisel]

This involves renaming the model class to SurvivalBoost as a short and easy to remember name.

[ogrisel]

@juAlberge @Vincent-Maladiere @judithabk6 I think this branch is in a good enough shape to be reviewed for merging.

• it exposes the new multi-incidence estimator with the new SurvivalBoost as decided during the sprint,
• it updated the tests and example to match the new API;
• it removed things that are not covered by the tests;
• it hides the IPCW estimator API to keep it private for now. I plan to open a dedicated PR to implement the generic wrapper we discussed with @judithabk6 at the sprint but this is better done in a dedicated PR.

Note: I also updated the marginal competing risk estimation example to use KM based IPCW as this example is a case of a marginal censoring mechanism. Using KM-IPCW really improves the quality of the plots on censored data. Even when tweaking the other hyper-parameters I could not get the alternating IPCW strategy to work well on this example. This will also be investigated later.