Provide a SLURM-free option

It should be relatively straightforward to provide a SLURM-free option, where

• no slurm jobs are created
• all the CI jobs are run on the local node
• no slurm jobs are terminated.

This would allow this project to provide all kind of functionality we want to provide for CI, and supersede ALL custom executors we provide.

In such a scenario, the custom executor queue would not be needed either. We want to retain the ability to use different containerisation backends, though.

A possible roadmap

• create a slurm-free version of the executor, while still keeping the custom executor queue (which would be launched on the local node). This will require some refactoring and changes in configure-slurm.sh (which should possibly be named differently - one needs to decide how much of that functionality would be common to the non-slurm version, for example).
• for the slurm-free version, remove the custom executor queue.