purging experiments while running does not cancel them (slurm launcher)

[bernlu]

When launching a run through slurm and then (while running) purging the run, the slurm job stays active. Simex does not show this and the slurm job then needs to be stopped via slurm commands - and the user has to know about this, otherwise there may be a lot of orphan jobs in the slurm queue.

Simex should probably warn the user and/or stop any running slurm tasks on purge.