docs: def make_snapshot(self):
          """Output a snapshot of the current model state, as a pickle of the
          `Model` object in a file inside the output directory, with a name
          determined by its iteration number.
          """
          filename = join(self.output_dir, '{:010d}.pkl'.format(self.model.i))
          output_utils.model_to_file(self.model, filename)