docs: def tseries(self):
          """Time series data.
  
          This is a :class:`pandas.DataFrame` with istep as index and variable
          names as columns.
          """
          if self._stagdat['tseries'] is UNDETERMINED:
              timefile = self.filename('TimeSeries.h5')
              self._stagdat['tseries'] = stagyyparsers.time_series_h5(
                  timefile, list(phyvars.TIME.keys()))
              if self._stagdat['tseries'] is not None:
                  return self._stagdat['tseries']
              timefile = self.filename('time.dat')
              if self.hdf5 and not timefile.is_file():
                  # check legacy folder as well
                  timefile = self.filename('time.dat', force_legacy=True)
              self._stagdat['tseries'] = stagyyparsers.time_series(
                  timefile, list(phyvars.TIME.keys()))
          return self._stagdat['tseries']