Create folds for k-fold validation

Used to create balanced folds with respect to y input for k-fold validation.

.createFolds(y, k = 10, list = TRUE, returnTrain = FALSE)

Arguments

y	split sample by balancing y
k	number of folds
list	logical whether to return folds in a list
returnTrain	logical whether to return training indices (T) or the test samples (F)