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

Is it overkill to document the ATD files a little? I'm not totally clear on the respective roles of created vs. running. Is a job first created and then converted into a running? Does a job have both created and optionally a running value? Maybe it's worth explaining a little about PBS here?