docs: def r_edges(step):
      """Cell border.
  
      Args:
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
      Returns:
          tuple of :class:`numpy.array`: the position of the bottom and top walls
          of the cells. The two elements of the tuple are identical.
      """
      rbot, rtop = misc.get_rbounds(step)
      centers = step.rprof.loc[:, 'r'].values + rbot
      # assume walls are mid-way between T-nodes
      # could be T-nodes at center between walls
      edges = (centers[:-1] + centers[1:]) / 2
      edges = np.insert(edges, 0, rbot)
      edges = np.append(edges, rtop)
      return edges, edges