COSMIT refactor cross-validation strategies
Include LeavePLabelOut in refactoring
A further refactor
COSMIT Base class for KFold/StratifiedKFold validation
COSMIT make BaseKFold abstract
COSMIT Base class for [Stratified]ShuffleSplit
COSMIT make helpers private with underscore
COSMIT make BaseKFold private with underscore
Merge pull request #2055 from jnothman/cv_refactor

[MRG] refactor partitioning cross-validation strategies