docs: def advth(step):
      """Theoretical advection.
  
      This compute the theoretical profile of total advection as function of
      radius.
  
      Args:
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
      Returns:
          tuple of :class:`numpy.array` and None: the theoretical advection.
          The second element of the tuple is None.
      """
      rbot, rtop = misc.get_rbounds(step)
      rmean = 0.5 * (rbot + rtop)
      rad = step.rprof['r'].values + rbot
      radio = step.timeinfo['H_int']
      if rbot != 0:  # spherical
          th_adv = -(rtop**3 - rad**3) / rmean**2 / 3
      else:
          th_adv = rad - rtop
      th_adv *= radio
      th_adv += step.timeinfo['Nutop']
      return th_adv, None