each slurm_submit_workers invocation only submits one batch...

...even if multiple workers are started

By assuming that each worker gets its own node within a given call this change allows multiple workers to be started within a single sbatch call.  This is important in my situation because our cluster policies enforce a maximum number of simultaneously running batch jobs regardless of resources allocated.  Note that workers from separate slurm_submit_workers invocations may be running on the same node but this seems of little consequence.