Opustrainer support + Full unit test passing (including runner tests)

[varisd]

• TrainModelSteps now uses OpusTrainer instead of directly calling Marian.
• MergeStep was adjusted to be configurable to properly prepare authentic and synthetic datasets for BT training
• Slurm and Bash runner tests are now passing, including step cancellation and resubmision. Wider test coverage still might be needed.