implementing support for maxConcurrent jobs

Release notes

Added --maxConcurrent option to default argparser for job submission. This allows to restrict the number of simultaneously active (=queued or running) jobs.

Details

closes #104 (closed) This works for both, local scheduling (via LocalController) and actual submission. On some clusters the number of jobs per user which can be simultaneously queued is limited, hence this feature is also needed for such clusters.