Lua plugin for SLURM integration

[CamStan]

Add a SLURM SPANK lua plugin to create an option for SLURM to call UnifyFS.

Option should start and stop the UnifyFS daemon as the user at job startup and termination.

Initial attempt (PR #308) made the assumption that 'unifyfsd' is installed in a default system path. The SPANK functions such as task_init and task_exit only run as the user running the job in a remote context.

SPANK Docs: https://slurm.schedmd.com/spank.html

Can test with a local SLURM install, otherwise may need someone to place the .lua scripts on some nodes for testing.
Can be invoked in the manner of: [srun|sbatch|salloc] -p pdebug --reservation=unifyfs …