Use --external-launcher for Slurm launch; fallback to old behavior; add srun wrapper CI job

[nchaimov]

This contains the commit from #58, which uses --external-launcher with the Slurm srun wrapper, with the following changes:

• Rebased on devel so that we have the CI tests in place.
• Falls back to old behavior for Slurm before 23.11 (when --external-launcher was added).
• Adds new CI job which builds Spindle with rm=slurm and without rshlaunch, in order to test the changed code.

[mplegendre]

I'm not sure another CI set was necessary for just this mode. But since the work is already done, I'm not going to object either.

Otherwise looks good.