Support PBS schedulers

[SvenMarcus]

In order to be useful on more HPC systems we should support PBS based schedulers besides Slurm.
This requires additional options in the configuration file as well as the addition of a new PBSController equivalent to the SlurmController.

[SvenMarcus]

Considerations for testing: currently we have a single assertion making sure that a job was started for slurm. Since the test cases stay the same for PBS based systems, but only the assertion step changes, we need to either introduce an assertion strategy or mocks that can assert the correct behavior themselves.