Allow MapieRegressor to use any split strategy

[thibaultcordier]

Is your feature request related to a problem? Please describe.
Split-CP is only available with "split" keyword or ShuffleSplit class of scikit-learn.

Describe the solution you'd like
Propose more compatible split BaseCrossValidator/BaseShuffleSplit.

Describe alternatives you've considered
In practice, we need to use the predictions aggregation for cross-CPs and not for split-CPs. All you need to do is find out the number of splits using the get_n_splits method to know whether it's a split or a cross.