Support `ntasks` in `SlurmExecutor`

[activatedgeek]

Is your feature request related to a problem? Please describe.

The current JobGroup relies on the first executor task to build arguments for sbatch.
It would be convenient to support the sbatch --ntasks argument so that the resource allocation calculation is a little easier than assigning --ntasks-per-node.

Describe the solution you'd like

Add ntasks to SlurmExecutor.

Describe alternatives you've considered

The alternative is to manually calculate potential topology of tasks per node based on the list of executors added to the experiment.

[CodersAcademy006]

@activatedgeek sent this as #589. Turned out to be smaller than it looks: SlurmExecutor.SBATCH_FLAGS already whitelists ntasks and SlurmBatchRequest builds its flags from asdict(executor) filtered through that list, so the only thing missing was the field itself, which is why additional_parameters={"ntasks": 8} half worked while still emitting --ntasks-per-node=1 from the field default next to it. The PR adds ntasks and drops ntasks_per_node whenever it is set, in both the sbatch build and the interactive srun helper, since Slurm treats --ntasks-per-node as a per-node maximum when --ntasks is present and the default of 1 would otherwise cap the request. Heterogeneous jobs are rejected rather than silently overridden, because those size each group through resource_group. I have no Slurm cluster to submit against, so the tests assert on the rendered sbatch script and the srun command line, and if you can try it on yours I would like to know whether the resulting allocation matches what you expected for your JobGroup case.