HPC: add LDAP & SLURM group for people using HPC for class

[bzh-bzh]

@glee- suggested adding a separate group for people who are using HPC for short-term classwork, so in the future (if necessary), we can lower their resource limits, or automatically trim them after each semester.

This script has to be generalized, and a new LDAP group created.

Optionally, it would be nice to make add_slurm_users create the SLURM accounts if they don't exist, so we get better bootstrapping.