Report URL through Slurm

Since the URL of the service is determined on the fly from the node address and the reserved ephemeral port, it must be communicated to the user. Currently this is done using a hidden file in the JUPYTER_NOTEBOOK directory. In order to create that file, the Slurm script needs to copy the output stream, parse it, and write a file in a predetermined location.

It would be simpler if the Slurm script could write that URL to the job metadata, in the Comment field for example. Clients could then retrieve the address using squeue -j JOBID -O Comment:0