Default partition, account, and cluster

[gvegayon]

slurmR should identify whenever the job is being run within a job. If that's the case, the following variables should have the following defaults:

• partition = $SLURM_JOB_PARTITION
• account = $SLURM_JOB_ACCOUNT
• cluster = $SLURM_CLUSTER_NAME

This way, users can skip writing this information twice when possible.