prtDataInterfaceCategoricalTargets
   Super class for data sets with discrete number of possible labels.
 
   Handles all logic involving classn names, class indices, etc.,
 
   Also enables use of NAN as unlabeled data; use
      hasUnlabeled
      retainUnlabeled
      removeUnlabeled
      retainLabeled(obj)
      removeLabeled(obj)
 
   to handle unlabeled observations.
 
   Note: NAN labels do not count towards the nClasses and will
   not appear in uniqueClasses.  It is not possible to set the name of
   the class corresponding to NAN.