code: def _labelToCategoryNumber(self, label):
      
      if label not in self.saved_categories:
        self.saved_categories.append(label)
      return pow(2, self.saved_categories.index(label))
docs: Since the KNN Classifier stores categories as numbers, we must store each
      label as a number. This method converts from a label to a unique number.
      Each label is assigned a unique bit so multiple labels may be assigned to
      a single record.