Path stuff...

Started working out how I'll locate, match and identify significant
paths in a molecule.  Hashed out (roughly) what I think the tests should
look like - there are a few places where I'm already foreseeing some
issues.  This will probably start as a massive function with only a few
helper functions, and then I'll refactor it into a lot of smaller
functions.  Also started writing the specs for this.  It's pretty basic
but should keep me on task