docs: def _post_read_flds(flds, header):
      """Process flds to handle sphericity."""
      if flds.shape[0] >= 3 and header['rcmb'] > 0:
          # spherical vector
          header['p_mesh'] = np.roll(
              np.arctan2(header['y_mesh'], header['x_mesh']), -1, 1)
          for ibk in range(header['ntb']):
              flds[..., ibk] = _to_spherical(flds[..., ibk], header)
          header['p_mesh'] = np.roll(
              np.arctan2(header['y_mesh'], -header['x_mesh']) + np.pi, -1, 1)
      return flds