docs: def _make_3d(field, twod):
      """Add a dimension to field if necessary.
  
      Args:
          field (numpy.array): the field that need to be 3d.
          twod (str): 'XZ', 'YZ' or None depending on what is relevant.
      Returns:
          numpy.array: reshaped field.
      """
      shp = list(field.shape)
      if twod and 'X' in twod:
          shp.insert(1, 1)
      elif twod:
          shp.insert(0, 1)
      return field.reshape(shp)