Binder notebook build broken

[jeffjennings]

There's a link in the repo README to launch a Jupyter instance with Binder in order to run the tutorial notebooks remotely. This fails (see #615) because the root of the repo no longer has a requirements.txt file or another configuration file, as expected by Binder.

The repo needs a configuration file added for Binder (see instructions) that tells it to look for the unique requirements.txt file in each directory in tutorials. (Or another solution to specify requirements across all notebooks.)