Submit multiple jobs for subsequent execution

[SteffenRenisch]

Implement mechanism to submit multiple jobs to be executed subsequently without any further manual intervention

• check whether more recent version of NVFlare offers features supporting this

[oleschwen]

For the time being, we want to have the option for manual intervention and checking if clients have succeeded/are still available.

[SteffenRenisch]

There is a parameter at startup of the server that sets a limit to the number of jobs worked on in parallel; setting it to 1 should make sure the submitted jobs are run subsequently. Has to be tested and confirmed.