Make cookbook materials fetchables

[dwhswenson]

Follow-up on #84: There should be a single source of truth for cookbook input files (SDFs, PDBs, etc.) instead of having a copy in, e.g., the OpenFE test suite, with a duplicate stored here.

We could then make the first line of each notebook be ! openfe fetch cookbook-files — this could be either a fetchable per-notebook or a single fetchable to download all cookbook notebooks.

This would have the advantage that even a download of a single notebook would be self-contained, instead of requiring users to download a tarball/clone of the repo.

Steps to make this happen:

1. Create the fetchables in the OpenFE CLI. See fetchables.py for examples
2. Cut a release of OpenFE, update ExampleNotebooks to use that OpenFE version.
3. Update the cookbook notebooks here to download the required files.