Better data handling for datasets

[cle-ros]

Is your feature request related to a problem? Please describe. There is no clean unified interface for datasets.

Describe the solution you'd like A unified interface should include the following properties:

• downloading data
• extraction of archives
• flattening data folders, or otherwise creating uniform folder structures

Replaces, for now, the following PRs: https://github.com/asappresearch/flambe/pull/259 https://github.com/asappresearch/flambe/pull/258

[jeremyasapp]

100% I plan a rework of the dataset + field modules soon