[MRG+1] add shuffle paramater to train_test_split (#8845)

* add shuffle paramater to train_test_split

* fix syntax error

* fix variable name

* fix formatting in doctest output

* fix doctest output

* refactor shuffle paramater into ShuffleSplit and StratifiedShuffleSplit

* include shuffle option in tests

* rollback refactor

* revert to simpler version of unshuffled split

* fix flake8 errors

* revert changes to ShuffleSplit

* revert BaseShuffleSplit

* more reversions

* fix indentation

* remove shuffle parameter from CVclass

* add text to NotImplementedError

* change indexing to use numpy.arange rather than range

* specify precondition for stratify to be None in docstring