Download engine modifications

This is a proof of concept. I tested it with a few datasets and will test it more comprehensively, but here it is so you can take a look as well.

* "retriever download" is now a shortcut for "retriever install download"

* The download only engine has only the engine methods that download files; all other methods (creating tables, inserting data, etc.) will do nothing. This means that the engine works for any script and just downloads all files, skipping every other step.

* After running, the download only engine copies all raw data files into the home directory. (This location could be customized easily by adding an argument to the engine.)