docs: def summarize_classes(self):
          """
          Summary of classes: names, numeric labels and sizes
  
          Returns
          -------
          tuple : class_set, label_set, class_sizes
  
          class_set : list
              List of names of all the classes
          label_set : list
              Label for each class in class_set
          class_sizes : list
              Size of each class (number of samples)
  
          """
  
          class_sizes = np.zeros(len(self.class_set))
          for idx, cls in enumerate(self.class_set):
              class_sizes[idx] = self.class_sizes[cls]
  
          # TODO consider returning numeric label set e.g. for use in scikit-learn
          return self.class_set, self.label_set, class_sizes