fixed bug in prtDataInterfaceCategoricalTargets.m for cases where:
1) prtClass* output incomplete prtDataSet, and we have to do something to infer
2) The number of classes in a test fold was not the same as the number of classes total (nFolds > #instances(class))

May have affected KNN in X-Val for cases where #Folds > number of observations in any class
fixed KNN to output complete dataset