Add octree support
- Refactor `Cell.js` to extend `THREE.Mesh`
- Refactor `Ecosystem.js` to spawn new cells on updates
- Add octree updating and rebuilding to `Ecosystem.js`
- Add octree node search and object intersecting logic to `Ecosystem.js`
- Refactor `main.js` to clal ecosystem update methods
- Stub additional camera option to pan around scene