Add desi_purge_tilepix

Add a script `desi_purge_tilepix` to purge all healpix-based redshifts (zpix) that use data from a tile. Implement that as a function that can be called by `desi_purge_tilenight` so that purging a tile can also purge its downstream zpix, just as it currently purges its downstream ztile outputs.