RFC: Interpolate Highways

Closes #29.

@tedsteiner This needs to be rebased once #26 is merged -- I'll take care of that then. Also, I'm sure I'll work on other functionality, and it would be easier if master kept up (apart from wanting to push any fixes to METADATA so other users can take advantage) -- I appreciate that you're busy and the package isn't part of your current work -- perhaps you could give me a commit bit?