Devise Resource Recommendations for cao

cao allows the user to specify --parallel, allowing multiple tests to be run concurrently. Figuring out the requirements for various parallel levels would be very nice to include in the README/ --help text, and would also mean we could optimise our test clusters.

Ideally, we'd be able to provide a general guideline (e.g. require parallel * 6 CPUs) so users can work out what fits them best.

This will probably mean running tests and keeping an eye on resource consumption so we can tweak appropriately.