Make DownloadData be idempotent

[duetosymmetry]

DownloadData can skip downloading if the requested data already exists. That way it's safe to put a call to DownloadData at the beginning of a script, without the user doing any checks. This just requires testing the destination path's .exists(). Example (with an optional flag overwrite):

https://github.com/duetosymmetry/qnm/blob/9008d69b22d7bb708886cf8f5ffeef5fb5a29bf4/qnm/cached.py#L504-L534