Iterators+Generators, plus a proposed new organization

I'm super excited at the growth of our repo on various data science projects; I'm starting to think it needs some structure in order to be more approachable. I've added the newest session on iterators and generators to a base directory called ``intermediate-python``. As a first attempt at organization, I'm imagining that there is a separation of "what level of experience am I looking for?", and "what language or environment am I looking for?", and possibly also "what kind of thing am I working on?" (eg machine learning algorithms, vs how-tos on databases).

Though I didn't make the distinction in this PR, the ``intermediate-python`` directory could also have subdirectories that keep e.g. any input data or modules cleanly separated. Another possible approach is to have ``data/`` and ``code/`` directories at the base that we can point to from within individual sessions/projects. 

I'm open to other ways of organizing this repo, but I would like to see some sort of easier method of entry for someone new who finds it. Feel free to provide comments on this PR and I can modify as needed.