Use a persistent location for caching data files

[nikhilwoodruff]

Right now the cache we use for data is reset on termination of the python process. This means data downloads are repeated every time a process is restarted. We could instead set a persistent location for the cache (since we're making sure we don't reuse outdated data by checking CRCs) so that we can avoid repeated downloads.

[anth-volk]

@nikhilwoodruff Before I review, could you define what you mean by "Python process"? The specific run of a Simulation's calculation method within one container context? More broadly, the initialization and persistence of a container? Or something else?