add multiplayer mode for coupled runs

[orichters]

Purpose of this PR

• REMIND-MAgPIE cascades with more than 4 scenarios cannot be run on priority slots by a single user, given the PIK cluster limitations. This helper scripts allows to run a single cascade with multiple users.
• Simply set qos to multiplayer, start your runs, and you and one or more colleagues run Rscript scripts/utils/multiplayer.R
• If the current user running a run has no priority free, the sbatch command that needs to be executed is pasted into scripts/utils/multiplayer.sh. The multiplayer.R scripts checks whether there are commands to be executed and how many priority slots are free, and then runs as many as possible. Afterwards, it starts a slurm job that will do the same 15 minutes later.
• Use --mem=8000 option only if running on 1 CPU. I asked in the RSE channel a while ago whether adding it would harm, and it does:

slurmstepd: error: Job 26812886 exceeded memory limit (8344256 > 8192000), being killed
slurmstepd: error: Exceeded job memory limit
slurmstepd: error: *** JOB 26812886 ON cs-f14c02b10 CANCELLED AT 2023-06-16T00:37:19 ***

Type of change

• New feature
• Requires documentation update (not done yet)

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I explained my changes within the PR, particularly in hard-to-understand areas
• I checked that the in-code documentation is up-to-date
• All automated model tests pass (FAIL 0 in the output of make test)

[dklein-pik]

Thanks, Olli, for this cool feature! Found one thing, see comment.

[orichters]

Not desired as discussed with @dklein-pik and @LaviniaBaumstark.