cleanup cluster.slurm.cheaha.json

release 3 - improve commenting and remove unused attributes in cluster.slurm.cheaha.json

• remove un-used and confusing 'cpus-per-task' attribute from default rule resources; replace with documentation on where threads/cpus are actually set, and add documentation for 'ntasks' attribute
• Add %x=job_name to default log name, to match current module code on cheaha; add URL to documentation on SLURM filename_pattern variables: https://slurm.schedmd.com/sbatch.html#lbAH