ENH: cleanup empty cache directory after a successful call to load_sample

[neutrinoceros]

PR Summary

Context: internally, yt.load_sample uses pooch to retrieve data from the hub server. Because data is obtained most often as compressed archives, we first download it to a temporary directory (yt_download_cache). This is an implementation detail that should not be directly visible to users.
This PR does two things:

• clean up the directory after it's served its purpose (this doesn't prevent pooch from re-creating it during the same session if needed)
• prepend a . to its name so its "hidden" in the conventional POSIX sense

[matthewturk]

I don't think we should hide it -- dumping a couple gigs into a hidden directory is not something I really want to push. That being said, does this mean that if I use yt.load_sample multiple times in multiple sessions, each time it will download afresh?

[neutrinoceros]

dumping a couple gigs into a hidden directory is not something I really want to push.

You have a point, I'll revert that part.

does this mean that if I use yt.load_sample multiple times in multiple sessions, each time it will download afresh?

it doesn't ! when the untared data is already on disk, load_sample doesn't download anything. This dir is only for tarballs, which we already auto-remove when untaring is successful.

[matthewturk]

it doesn't ! when the untared data is already on disk, load_sample doesn't download anything. This dir is only for tarballs, which we already auto-remove when untaring is successful.

Great -- for some reason I thought it ran a hash on the tarball to see if it needed to re-download, rather than looking at contents of the directory.

[neutrinoceros]

pooch internally verifies that the file's hash matches the data base we provide for security, that might be where you got that idea :)

[chrishavlin]

looks good to me.