add light trap assignments

[sbailey]

The fiber positioners will have light traps along their arms, to be used for capturing the light of especially bright objects instead of letting their light randomly scatter about the instrument.

• define target bit(s) for targets that should be placed on a light trap (this actually goes into desitarget, but fiberassign needs to know about these bits as a special case)
  • some will be so bright that putting them on a light trap is higher priority than science targets
  • others may be lower priority
• add fiber assignment code that knows how to move a positioner to align the light trap with the target instead of aligning the fiber with the target. Coordinate with Joe Silber to understand the geometry.

[sbailey]

FYI, DESI-0899 is the doc with the definition of where the light traps are on the ends of the positioners.

[tskisner]

Revisiting this issue in light of recent code developments.

• Adding an additional "trap" target category in fiberassign (currently: science, standard, sky, and "safe") is straightforward to do, and would allow adding targets to the catalogs with bits set to indicate they are of "trap" target type. Assigning "trap" targets could be done first, before assigning science targets.

• For each positioner, we would need to include the locations of the light traps in the hardware model. See https://github.com/desihub/desimodel/issues/106

• The final piece needed is the one that is a little more involved. We would need to add a boolean switch at several places in the code to choose between assigning to the fiber or assigning to the trap. We can't just dynamically change the arm length, because we need to consider the full positioner when checking for collisions.

[sbailey]

To first order, DESI_TARGET bit 50 "BRIGHT_OBJECT" would be the things we'd consider for putting on a light trap.