Downloads are cached as .rds files so that a panel can be rebuilt
offline and so that a data vintage stays pinned until you refresh it.
rps_cache_dir() returns the cache directory, rps_cache_list() lists
cached downloads and rps_cache_clear() deletes them.
Value
rps_cache_dir(): a path. rps_cache_list(): a data frame with
file, source, retrieved and size_kb. rps_cache_clear(): the
number of files removed, invisibly.