off-footprint stuck skies check in assemble_fibermap

[sbailey]

If/when the backup program tiles use desitarget.skyhealpix (desihub/desitarget#771) to get off-footprint sky locations, that will fail in desispec.io.fibermap.assemble_fibermap that double checks that those stuck positioners are still on sky using desitarget.skybricks (and thus will decide they aren't on sky): Currently this is desispec/py/desispec/io/fibermap.py starting at line 705, but could move a bit after PR #1459 is merged:

https://github.com/desihub/desispec/blob/master/py/desispec/io/fibermap.py#L705

        #- for SKY on stuck positioners, recheck if they are on a blank sky
        #- (e.g. they might be "off" target but still ok for sky)
        log.info('Checking if SKY on stuck positioners are still on SKY locations')
        tilera = fa.meta['TILERA']
        tiledec = fa.meta['TILEDEC']
        tileradius = get_tile_radius_deg()
        skybricks = Skybricks()
        ok = skybricks.lookup_tile(tilera, tiledec, tileradius,
                fibermap['FIBER_RA'][stucksky], fibermap['FIBER_DEC'][stucksky])
...

[araichoor]

I just submitted a fiberassign PR on this topic: desihub/fiberassign#416.
As written in the PR description, I ve added in the fiberassign-TILEID.fits.gz header a LKSKYSRC keyword (for "lookup_sky_source"):

• if LKSKYSRC=="ls", then one should use $SKYBRICKS_DIR,
• if LKSKYSRC=="gaia", then one should use $SKYHEALPIXS_DIR, which also has to be defined.

Then you'd have to implement in desispec fibermap.py the same kind of switch as in https://github.com/desihub/fiberassign/blob/299e10a877797dc27d77386acad43698fc6263fc/py/fiberassign/stucksky.py#L96-L117.

Let me know if you'd like the approach to be different.