[MRG+1] Repeated K-Fold and Repeated Stratified K-Fold (#8120)

* Add _RepeatedSplits and RepeatedKFold class

* Add RepeatedStratifiedKFold and doc for repeated cvs

* Change default value of n_repeats

* Change input parameters of repeated cv constructor to n_splits, n_repeats, random_state

* Generate random states in split function rather than store it beforehand

* Doc changes, inheriting RepeatedKFold, RepeatedStratifiedKFold from _RepeatedSplits and other review changes

* Remove blank line, put testcases for deterministic split in loop and add StopIteration check in testcase

* Using rng directly as random_state param to create cv instance and added a check for cvargs

* Fix pep8 warnings

* Changing default values for n_splits and n_repeats and add entry in changelog

* Adding name to the feature

* Missing space