Load averages

[lars-t-hansen]

(To be fleshed out)

Similar to #242, we want to exfiltrate the node load average as a proxy for the number of runnable jobs, to give an idea of imbalance between allocated resources and available work.

For a Slurm job we really want the number of runnable jobs / the load average for the job, in the context of its allocation. This may be very hard in the context of Sonar, but it may be easier if we use a slurm-centric extractor, see #187. This is all TBD and very speculative.