docs: def _save_results(self, output_dir, label, results, ngrams, type_label):
          """Saves `results` filtered by `label` and `ngram` to `output_dir`.
  
          :param output_dir: directory to save results to
          :type output_dir: `str`
          :param label: catalogue label of results, used in saved filename
          :type label: `str`
          :param results: results to filter and save
          :type results: `pandas.DataFrame`
          :param ngrams: n-grams to save from results
          :type ngrams: `list` of `str`
          :param type_label: name of type of results, used in saved filename
          :type type_label: `str`
  
          """
          path = os.path.join(output_dir, '{}-{}.csv'.format(label, type_label))
          results[results[constants.NGRAM_FIELDNAME].isin(
              ngrams)].to_csv(path, encoding='utf-8', float_format='%d',
                              index=False)