Take a pre-existing maglim map and divide it into
    chunks consistent with the catalog pixels.

def split(config,dirname='split',force=False):
    """ Take a pre-existing maglim map and divide it into
    chunks consistent with the catalog pixels. """

    config = Config(config)
    filenames = config.getFilenames()
    #healpix = filenames['pix'].compressed()

    # Check that things are ok
    basedir,basename = os.path.split(config['mask']['dirname'])
    #if basename == dirname:
    #    msg = "Input and output directory are the same."
    #    raise Exception(msg)
    outdir = mkdir(os.path.join(basedir,dirname))
    
    nside_catalog = config['coords']['nside_catalog']
    nside_pixel = config['coords']['nside_pixel']

    release = config['data']['release'].lower()
    band1 = config['catalog']['mag_1_band']
    band2 = config['catalog']['mag_2_band']

    # Read the magnitude limits
    maglimdir = config['maglim']['dirname']

    maglimfile_1 = join(maglimdir,config['maglim']['filename_1'])
    logger.info("Reading %s..."%maglimfile_1)
    maglim1 = read_map(maglimfile_1)
    
    maglimfile_2 = join(maglimdir,config['maglim']['filename_2'])
    logger.info("Reading %s..."%maglimfile_2)
    maglim2 = read_map(maglimfile_2)

    # Read the footprint
    footfile = config['data']['footprint']
    logger.info("Reading %s..."%footfile)
    footprint = read_map(footfile)

    # Output mask names
    mask1 = os.path.basename(config['mask']['basename_1'])
    mask2 = os.path.basename(config['mask']['basename_2'])

    for band,maglim,base in [(band1,maglim1,mask1),(band2,maglim2,mask2)]:
        nside_maglim = hp.npix2nside(len(maglim))
        if nside_maglim != nside_pixel:
            msg = "Mask nside different from pixel nside"
            logger.warning(msg)
            #raise Exception(msg)

        pixels = np.nonzero(maglim>0)[0]
        superpix = superpixel(pixels,nside_maglim,nside_catalog)
        healpix = np.unique(superpix)
        for hpx in healpix:
            outfile = join(outdir,base)%hpx
            if os.path.exists(outfile) and not force:
                logger.warning("Found %s; skipping..."%outfile)
                continue

            pix = pixels[superpix == hpx]
            print(hpx, len(pix))

            logger.info('Writing %s...'%outfile)
            data = odict()
            data['PIXEL']=pix
            data['MAGLIM']=maglim[pix].astype('f4')
            data['FRACDET']=footprint[pix].astype('f4')
            ugali.utils.healpix.write_partial_map(outfile,data,nside_pixel)