docs: def get_rbounds(step):
      """Radial or vertical position of boundaries.
  
      Args:
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
      Returns:
          tuple of floats: radial or vertical positions of boundaries of the
          domain.
      """
      if step.geom is not None:
          rcmb = step.geom.rcmb
      else:
          rcmb = step.sdat.par['geometry']['r_cmb']
          if step.sdat.par['geometry']['shape'].lower() == 'cartesian':
              rcmb = 0
      rcmb = max(rcmb, 0)
      return rcmb, rcmb + 1