Return the indices of the super-pixels which contain each of the sub-pixels.

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