Longitudinal benchmarking

libCEED should be able to track performance over time. The benchmarking tools should make it easy (one or two commands) to compare performance for a particular backend between any two commits. There should be both graphical and small human-readable output, with detailed CSV output available for further analysis.

It would also be useful to have low-dimensional summary stats so we can plot performance versus calendar-time for any given backend.

With #219 and a low-noise testing environment, we should set a CI job to run periodically (weekly might be good enough) on 'master' and log the results for future analysis.