restraints and constraints.
Moving sixDoFRigidBodyMotion back one directory.
Adding uncoupledSixDoFRigidBodyDisplacement to execute motion
specified by the six DoF motion state, but not applying surface
forces. Useful for pre-displacing a mesh.
Adding constrainedSixDoFRigidBodyDisplacement to temporarily perform
Vorticity's lander simulation. Will be removed when generalised
constraints are added.