Add requirements.txt file for tutorial notebook-specific dependencies?

[egpbos]

Following this PR in the exploration repo, I think we should also have a requirements.txt file here for tutorial notebook dependencies. This will allow the repo to be run on Binder.

If there's another way to get it to work on Binder without the requirements.txt file, just with the dependencies from setup.cfg, that would probably be preferable.

Related to #132.