Classes
 ClassDescription
 b2BuoyancyController Calculates buoyancy forces for fluids in the form of a half plane
 b2ConstantAccelController Applies an acceleration every frame, like gravity
 b2ConstantForceController Applies a force every frame
 b2Controller Base class for controllers.
 b2ControllerEdge 
 b2GravityController Applies simplified gravity between every pair of bodies
 b2TensorDampingController Applies top down linear damping to the controlled bodies The damping is calculated by multiplying velocity by a matrix in local co-ordinates.