Scheduler agnostic directives

[bschroeter]

Directives are currently handled through interpolation into a job script template. This is fine for the current target of PBS on Gadi, but down the track we may consider adding these in dynamically in order to take a single job script and make it work across different scheduling system (i.e. SLURM).

Of course, this may not be necessary, but could be a nice thing to have.

[bschroeter]

This will need a translation mapping of submission arguments between schedulers.