Generate GFA target catalog for each tile

[sbailey]

In addition to fiber assigned targets, we need to provide ICS with a list of every object that might be visible on the GFAs. These will be used for field acquisition, guiding, and focus.

desimodel.focalplane.get_gfa_targets and lower-level .on_gfa, and .on_tile_gfa provide routines for doing this. These need to be combined with desitarget and/or fiberassign into output files to pass forward.

To do:

• Define format. e.g. options include
  • an extra HDU of the fiberassignment files (easy and natural for interface with ICS, but mixes C++ and python code from separate repos...)
  • separate files so that GFA target identification can be developed and run separately from fiberassignment
• Write wrapper code to actually do that.

This isn't particularly hard since desimodel.focalplane.get_gfa_targets already exists, but finalizing the details is absolutely a blocking factor for first light.

[sbailey]

Update: the format output by select_gfas in PR #342 is good, e.g. /global/cscratch1/sd/adamyers/gfas-dr6.0-PR342.fits (note: on $SCRATCH, will not be around forever). We still need a standalone wrapper or instructions for using that to generate the subset that actually overlaps real GFAs. fiberassign does that subselection but doesn't propagate all columns. Adding this to 18.9 project and leaving it open until we have that wrapper.

[sbailey]

This is included in fiberassign output now. Closing ticket.