Load datasets from a standard location

[nightlark]

Downloaded datasets used by our tools should be stored in a standard location (XDG_DATA directory on POSIX systems?), and we should include a file to keep track of any additional metadata about the current downloaded version that may be useful (version identifier, hash, etc).

Having a standard location will also make so other tools that use the dapper datasets know where to look, and don't have to maintain their own duplicate copy of the same dataset.

[nightlark]

Similar to: https://github.com/LLNL/Surfactant/blob/main/surfactant/configmanager.py#L145-L156 and https://github.com/LLNL/Surfactant/blob/main/surfactant/configmanager.py#L145-L156