Implement a user cache dir

Should probably default to `platformdirs.user_cache_dir("drevalpy")`, which resolves to `~/Library/Caches/drevalpy` on Mac and `~/.cache/drevalpy` on Linux. Should be overridable via `DREVALPY_CACHE_DIR` environment variable (following the example of other libraries, such as matplotlib and numba).

This should be the place where all the downloaded datases and the `meta` data are stored. Currently, the default directory to store the data is the working directory, making it necessary to explicitly provide a system-wide data directory on every run. With the environment variable, it would be a one-time setting.

The issue does not name files or tests; first locate the entry points that choose storage for downloaded datasets and metadata. Check how other configuration environment variables are handled, then verify the platformdirs default and DREVALPY_CACHE_DIR override on supported systems. Done means both data types use the selected cache directory without requiring a working-directory setting.

Written by the indexing model from the issue text.