Pbs backend + bug fix

The bug-fix is a one-line change.

The PBS backend is a big commit which includes tests and all.
The test workflow(s) are a bit fragile: some failure trigger the clean-up too soon. But it is a testing problem; they should not block the PBS backend.

EDIT: grammar

Yes Torque (PBS implementation) has 3 daemons + 1 client.

I think the Mom is the server that is on the computing nodes, and it waits for orders from the scheduler (I may be wrong, it is quite a mess).