Twang

The main addition here is the integration of Demucs source separation, their latest model which works fantastically well for isolating guitar. Tutorial to follow soon!

Kicks off the project by setting up some of the basic frameworks
- `BaseTrack`, `PydubTrack`, & `LibrosaTrack` implementations; standardised APIs for representing audio tracks
- The `SourceSeparation` class represents the API for doing audio source separation, with the first two specialisations included: nussl & demucs.