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.

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)