Replace old tutorials with Jupyter notebooks

Created by: gcarleo

The tutorials on the website should be replace by Jupyter notebooks, and should be part of the main NetKet repository. This has several advantages:

1. ipython notebooks are easily converted to markdown, thus can be immediately put on the website
2. both the documentation and the tutorials will leave on the main repo, thus being updated more regularly with PRs and also being tested more efficiently

@kchoo1118 has a very nice example of how the tutorials should be, please do a PR with it and let's help him finish the other tutorials!