docs: def get_cycle_numbers(self, dataset_number=None, steptable=None):
          """Get a list containing all the cycle numbers in the test."""
          if steptable is None:
              dataset_number = self._validate_dataset_number(dataset_number)
              if dataset_number is None:
                  self._report_empty_dataset()
                  return
              d = self.datasets[dataset_number].dfdata
              cycles = np.unique(d[self.headers_normal.cycle_index_txt])
          else:
              cycles = np.unique(steptable[self.headers_step_table.cycle])
          return cycles