Use astropy cache/data infrastructure throughout

[eteq]

This needs some thought about which things should be cached, but in general the astropy data downloading framework should be used

[keflavich]

Could you post up a link to said framework (when available) ?

[eteq]

If you look at http://docs.astropy.org/en/latest/configs/index.html#functions, you'll see a set of functions of the form get_data_* - those transparently use the cache. In particular, you probably want either get_data_contents or get_data_fileobj, which accept a URL, and either give you the downloaded file or the cached version. Now that you mention it, there should probably be some mention of this in the narrative documentation, but the function docstrings pretty much tell you how to use it.

[keflavich]

I believe this is redundant with issue #23 and addressed by PR #28 now - @astrofrog,@eteq, can you confirm that I've used the functions correctly?

[eteq]

I think you're right @keflavich - so once #28 is merged (or similar), we can close this, but not until then.