prtDecisionBinary Base class for all prtDecisionBinary objects
 
  A prtDecisionBinary object is an abstract class and cannot be
  instantiated.    
 
  prtBinaryDecsion objects find a threshold value that is used to make
  decisions based on certain criteria.
 
  prtDecisionBinary objects all have the following function:
 
  getThreshold - return the prtDecisionBinary objects decision
                 threshold