extend maybe_download_and_extract to manually downloaded data sets

[dustinvtran]

It will only extract the files. Basically, extend the function to include url=None.

[dustinvtran]

Note: This is important for using the Observations library for restricted data sets, where you might first need to ask the author or go through some process to download the data set yourself. For example, imagenet or bookcorpus.