code: def qhalf(self):
          
          if self.__qhalf != None:
              return self.__qhalf
          self.log("qhalf")
          self.__qhalf = self.obscov ** (-0.5)
          self.log("qhalf")
          return self.__qhalf
docs: get the square root of the cofactor matrix attribute. Create the attribute if
          it has not yet been created
  
          Returns
          -------
          qhalf : pyemu.Matrix