docs: def mobility(sdat, tstart=None, tend=None):
      """Plates mobility.
  
      Compute the ratio vsurf / vrms.
  
      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`: mobility and time arrays.
      """
      tseries = sdat.tseries_between(tstart, tend)
      steps = sdat.steps[tseries.index[0]:tseries.index[-1]]
      time = []
      mob = []
      for step in steps.filter(rprof=True):
          time.append(step.timeinfo['t'])
          mob.append(step.rprof.iloc[-1].loc['vrms'] / step.timeinfo['vrms'])
      return np.array(mob), np.array(time)