API change the shape of the incidence curves

[glemaitre]

Change some API such that the shape of returned arrays are:

• predict_proba is an array of shape (n_samples, n_events)
• predict_cumulative_incidence is an array of shape (n_samples, n_events, n_times)
• predict_survival_function is an array of shape (n_samples, n_times)

[ogrisel]

The goal is to be consistent with the conventions of scikit-learn where all predict_ methods return datastructures with a first dimension that is aligned with the observations onto which the predictions are made.