code: def superpixel(subpix, nside_subpix, nside_superpix):
      
      if nside_subpix==nside_superpix: return subpix
      theta, phi =  hp.pix2ang(nside_subpix, subpix)
      return hp.ang2pix(nside_superpix, theta, phi)
docs: Return the indices of the super-pixels which contain each of the sub-pixels.