Prototype System class.

@moorepants, can add your tests to this branch? From this point I'll write code in a test-driven way. It'd be fun to continue to work on this together, but I'm also happy to do the coding and you can review afterwards (if you're busy, etc.).

- [ ] Edit Scene so that it takes in a System.
- [ ] Make KanesMethod and LagrangesMethod derive from a single abstract base class (ABC), perhaps MechanicsMethod. This ABC will be required to provide certain attributes.
- [ ] Document in Sphinx, README, etc.
Yes, that is why we are creating this, so that you can have something easier to work with in your viz code. It won't be ready tomorrow, but please help contribute to this so we can get it done more quickly.