Add option for limiting number of jobs running at once

[tlangfor]

Slurm provides a way to meter or limit the number of concurrently running jobs like this:

--array=0-100%10

which will only let 10 jobs run at the same time.

There are some workflows that tend to be IO heavy and could benefit from being able to add a --limit=10 or similar command-line option.

[brevans]

dSQ handles this with the --max-jobs option. Maybe it should be named better?

[tlangfor]

🤦