Add documentation about array jobs

Should include:

• submit syntax (ranges, concurrency, etc)
• using $SLURM_ARRAY_TASK_ID
  • To select dataset (e.g., select a line from a file, a file/directory from a directory listing, etc)
  • As random number seed (for reproducible results)
• how it affects emails (generally avoid --mail-type=ARRAY_TASKS for big arrays)
• how to cancel individual id's and the entire array
• scontrol update jobid=.... arraytaskthrottle=N to update max concurrent