docs: def hdf5(self):
          """Path of output hdf5 folder if relevant, None otherwise."""
          if self._rundir['hdf5'] is UNDETERMINED:
              h5_folder = self.path / self.par['ioin']['hdf5_output_folder']
              if (h5_folder / 'Data.xmf').is_file():
                  self._rundir['hdf5'] = h5_folder
              else:
                  self._rundir['hdf5'] = None
          return self._rundir['hdf5']