Alternative pose integrators and custom gravity handling

[RossNordby]

At the moment, gravity is assumed to be a single simulation wide constant by the PoseIntegrator. Some simulations do not require gravity, and others may want to use per-body gravity of some form.

We can easily support the different use cases by letting the user supply a gravity callback in the same way that we support zero overhead narrowphase callbacks (generic structs). Since the pose integrator relies on body data in AOS format, it wouldn't be too terrible to rely on the user to provide the desired implementation.

The same sort of approach could be used to implement damping. The callback would effectively a general purpose velocity modifier.

[RossNordby]

b9f37d5 provides the necessary pose integrator callback.