Add a way to clear the sample data cache

[Cadair]

This is currently an issue because we have updated it.

I added this as the bare minimalist possible approach.

I would like a way for us to be able to push an update to the user tools and have the sample data cache be cleared for everyone. The best way I have thought to do this is have a date string stored in the _sample.py somewhere and if the write time on the .tar files is older than that date then we force a redownload.