fix max_velo_diff for archetypes

[sbailey]

This PR is a followup to #300 which set max_velo_diff for spectype=STAR differently than GALAXY and QSO, but didn't handle the archetype case where spectype is a list of strings instead of a single string. This test command used to fail but now passes with this PR:

srun -n 4 --gpu-bind=map_gpu:3,2,1,0 --cpu-bind=cores rrdesi_mpi --gpu -n 10 \
  -i $CFS/desi/spectro/redux/iron/tiles/cumulative/80605/20210205/coadd-6-80605-thru20210205.fits \
  --archetypes /global/cfs/cdirs/desi/users/abhijeet/new-archetypes/rrarchetype-galaxy.fits \
  --model $SCRATCH/redrock/rrmodel-test.fits \
  -o $SCRATCH/redrock/redrock-test.fits

I also verified that removing the --archetypes option and running in non-archetype mode also continues to work.

[coveralls]

coverage: 39.09% (+0.06%) from 39.033%
when pulling 4c845bc on velodiff_archetypes
into ae069fa on main.