kfolds  Perform K-folds cross-validation of prtAction
  
     OUTPUTDATASET = self.kfolds(DATASET, K) performs K-folds
     cross-validation of the prtAction object OBJ using the
     prtDataSet DATASET. DATASET must be a labeled prtDataSet,
     and K must be a scalar integer, representing the number
     of folds. kfolds Generates cross-validation keys by
     partitioning the dataSet into K groups such that the
     number of samples of each unique target type is held
     approximately constant.
 
     [OUTPUTDATASET, TRAINEDACTIONS, CROSSVALKEYS] =
     self.kfolds(DATASET, K)  outputs the trained prtAction
     objects TRAINEDACTIONS, and the generated cross-validation
     keys CROSSVALKEYS.
 
     To manually set which observations are in which folds, see
     crossValidate.

Help for prtPreProcLda/kfolds is inherited from superclass prtAction