propagate pre-calculated healpix pixel number

[sbailey]

Filing under desitarget because this is where it starts, but this is a cross repo ticket. From @weaverba137 in desihub/two_percent_DESI#6:

If healpix ids are known at the time of fiberassignment, it would be good to include those. It would possibly simplify some joins.

Target selection calculates this and outputs as HPXPIXEL column. This should be propagated through fiber assignment, fibermap files, expanded fibermap tables in spectra files, and included in the output zcatalog table.

Currently we recalculate the healpixel ID from RA,dec at multiple steps. This is reproducible as long as we stay within python and healpy, but it could become problematic if we ever branch beyond that and use a different healpix library that may have edge effect differences (AFAICT the healpix standard doesn't define which pixel an (RA,dec) belongs to if it lands exactly on a pixel boundary or corner).

[moustakas]

I propose we close this issue, since it's not really for desitarget. I created specific issues in
desihub/desisim#410 and
desihub/fiberassign#96 (comment) so we can track this there.

We probably also need an issue in redrock (or desispec?), to ensure the (same) healpixel number gets all the way to the end.

[moustakas]

I believe the healpix number is being correctly propagated, at least when it leaves desitarget and at through fiberassign. There's still an open issue in desisim (desihub/desisim#410) but we can dispense with that separately.