docs: def dtime(sdat, tstart=None, tend=None):
      """Time increment dt.
  
      Compute dt as a function of time.
  
      Args:
          sdat (:class:`~stagpy.stagyydata.StagyyData`): a StagyyData instance.
          tstart (float): time at which the computation should start. Use the
              beginning of the time series data if set to None.
          tend (float): time at which the computation should end. Use the
              end of the time series data if set to None.
      Returns:
          tuple of :class:`numpy.array`: dt and time arrays.
      """
      tseries = sdat.tseries_between(tstart, tend)
      time = tseries['t'].values
      return time[1:] - time[:-1], time[:-1]