Make cookbook materials fetchables

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.

Start with openfecli/fetchables.py for existing fetchable patterns, then inspect the cookbook notebooks and the duplicate input files mentioned in the issue. Coordinate the OpenFE release and version update in ExampleNotebooks before changing notebook setup lines. Done means the required cookbook inputs download through OpenFE and a single notebook works without a repository clone or tarball.