Investigate if worth staging/caching selected files

[eford]

Looking into possibility of caching some files to a local disk and/or a RAM disk.
(Chuck also mentioned something about Optane memory, but I don't know if that would be a good fit.)
The obvious candidates are:

• calc_ccfs: reads the day's L2 files, particularly once we start processing them with multiple configurations in one pass (rather than as separate jobs).
• calc_rvs: Currently reads the daily_ccfs_?.jld2 files. (In the future, we hope to have an alternative algorithm that would need access to the L2 files, rather than the daily_ccfs_?.jld2 files.)