Describe the code of the physics example.
Speed improvements in the collision loop.

- Use Boost.Graph to solve the process order of the items.
- Cache the bounding box of the items to prevent useless computations.
- Use a dummy shape to fake the implementation of bear::universe::shape.
- Pre compute the bounding boxes before searching the collisions.
- bear::universe::static_map caches the bounding box of the items.