Avoid using a direct adaptation of OneVsRestClassifier from scikit-learn in OneVsRestCalibrator

[perellonieto]

The current function OneVsRestCalibrator https://github.com/perellonieto/PyCalib/blob/master/pycalib/models/multiclass.py#L72 uses an adapted copy of the scikit-learn OneVsRestClassifier https://github.com/scikit-learn/scikit-learn/blob/95119c13a/sklearn/multiclass.py#L138

It would be good to clarify what parts are necessary for this code, and what parts can be refactored in a way that we can use directly scikit-learn without duplicating the code.