add job monitor for SLURM

[brendanf]

Prework

• I understand and agree to the Contributor Code of Conduct.
• I have already submitted a discussion topic or issue to discuss my idea with the maintainer.

Related GitHub issues and pull requests

• Ref: Monitor classes for SLURM, PBS, and LSF #32

Summary

Implements job monitor class for SLURM, based on the existing SGE monitor. Tests in tests/slurm/monitor.R passed on my SLURM cluster (after appropriate modifications to script_lines).

[wlandau]

This is fantastic, thank you so much for writing and testing this SLURM monitor! I only have one minor request (documented inline).

[wlandau]

Also, feel free to add yourself as an author or contributor in the DESCRIPTION file