add FIBER_RA/DEC; support unpositioned coords files

[sbailey]

This PR

• fixes assemble_fibermap bug for "unpositioned" exposures #1063 by updating assemble_fibermap to support coordinates files that don't have any positioner iterations in them, e.g. 20201220/69082
• propagates FIBER_RA and FIBER_DEC if present in the coordinates file (e.g. 20201222/69415), but don't crash if they aren't (e.g. 20200315/55611)
• Renames long keyword "FIBERASSIGN" (>8 char) in early data to match current header name "FIBASSGN" that won't generate HIERARCH warnings.

Tested with

assemble_fibermap -n 20200315 -e 55611 -o fibermap-00055611.fits
assemble_fibermap -n 20201220 -e 69082 -o fibermap-00069082.fits
assemble_fibermap -n 20201222 -e 69415 -o fibermap-00069415.fits

And also tested end-to-end with an "unpositioned" exposure 20201220/69082 with

desi_proc -n 20201220 -e 69082 --batch --traceshift --cameras b1,r1,z1,b2,r2,z2

Outputs in /global/cfs/cdirs/desi/users/sjbailey/spectro/redux/fatest.