desi_proc specex wrapper MPI rank bug for single camera

[sbailey]

When running desi_proc --batch on an arc exposure with a single camera, the generated batch script has the wrong number of MPI ranks leading to a specex wrapper failure:

Example:

desi_proc --traceshift -n 20210507 -e 87539 --cameras b5 --batch --nosubmit

Generates a script with

srun -N 1 -n 11 -c 5 desi_proc --traceshift -n 20210507 -e 87539 --cameras b5 ... -mpi

(for simplicity I dropped the full path to desiproc and the long --timing-file option)

Note that has 11 instead of 21 ranks. Running that causes:

...
Traceback (most recent call last):
  File "/global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/code/desispec/0.48.0/bin/desi_proc", line 7, in 
<module>
    proc.main(args)
  File "/global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/code/desispec/0.48.0/lib/python3.8/site-packages
/desispec/scripts/proc.py", line 464, in main
    desispec.scripts.specex.run(comm,cmds,args.cameras)
  File "/global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/code/desispec/0.48.0/lib/python3.8/site-packages
/desispec/scripts/specex.py", line 336, in run
    sc = Schedule(fitbundles,comm=comm,njobs=len(cameras),group_size=group_size)
  File "/global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/code/desispec/0.48.0/lib/python3.8/site-packages
/desispec/workflow/schedule.py", line 67, in __init__
    raise Exception("can't have group_size larger than world size - 1")
Exception: can't have group_size larger than world size - 1

@marcelo-alvarez please update the bookkeeping and test desi_proc --batch --nosubmit ... with various combinations of single cameras, N>1 random individual cameras, complete spectrographs, and all spectrographs to confirm that it generates the intended number of ranks.