Allow loading of DESI fiberassign files

[djschlegel]

Allow loading of DESI fiberassign files that use TARGET_RA,TARGET_DEC instead of RA,DEC.
Ideally, we would display information from the first three HDU's of these files:
HDU#1: 5000 fiber positions, perhaps use MORPHTYPE column for the name
HDU#2: 20 sky monitor fiber positions, could name as "SKYMON"
HDU#3: Hundreds of potential guide stars, although better if trimmed to be within the GFA area

Data model for these files here:
https://desidatamodel.readthedocs.io/en/latest/DESI_TARGET/fiberassign/tile-TILEID-FIELDNUM.html

Example files here:
http://www.astro.utah.edu/~u6022465/cmx/tiles/DITHERING/v4/Tile_63016/