fixes desi_zcatalog --fibermap for new format fibermap

[sbailey]

This PR fixes desihub/desisim#448, where desi_zcatalog --fibermap didn't work for the new format fibermap. This implementation works on zbest files with both old and new format fibermap tables, and adds columns RA, DEC, FLUX_G, FLUX_R, FLUX_Z in both cases, even though the old version used to write MAG instead.

Unlike the discussion in desihub/desisim#448 this morning, I actually have tested it this time, using the 18.7 (old) and 18.11 (new) reference runs in /project/projectdirs//desi/datachallenge/reference_runs/ .

@TEtourneau please take a look if you'd like. Otherwise I'll merge tomorrow morning.

[TEtourneau]

I tested desi_zcatalog --fibermap, and it works.
Thanks for fixing the bug!

Just a practical question, what's the best way to use different branchs in the desi environment ?
To use desisim and desispec, I use this command:
source /project/projectdirs/desi/software/desi_environment.sh master
But I didn't know how to test desi_zcatalog from the branch target_ra, so I just git clone in my home directory desispec and switched to the branch target_ra.

[sbailey]

Thanks for testing @TEtourneau . Here's what I do for testing branches:

#- Use pre-installed master versions of all packages (updated nightly)
source /project/projectdirs/desi/software/desi_environment.sh master

#- Remove desispec/master from my path
module unload desispec

#- Replace it with my copy
cd $SCRATCH/desi/code/desispec
git checkout target_ra
export PYTHONPATH=$(pwd)/py:$PYTHONPATH
export PATH=$(pwd)/bin:$PATH

I've actually wrapped this in my own module file so that I can module swap desispec/sjb to replace the standard version with my own, but it is equivalent to the above.