add option --slurm-sleep-between-submit

This adds a new user option for slurm, to tell pbilby to insert sleep commands between the sbatch instances for parallel analysis jobs (n_parallel>1). This can be useful when the cluster one's working on is a bit busy filesystem-wise: e.g. I've recently had several times jobs fail due to resource loading conflicts for either matplotlib caches or distance marginalization .npz files. I've also decided to put in verbose echo statements so that the user isn't confused when the submit script seems to "hang" after adding this option. Also, if the option is not set, nothing should change at all.