docs: def output_files(self):
          """Return list of output files to be retrieved"""
          output_list = []
  
          pm_dict = self.get_dict()
          output_list.append(pm_dict['output_surface'])
          if pm_dict['target_volume'] != 0.0:
              output_list.append(pm_dict['output_surface'] + str(".cell"))
  
          return output_list