sixDoFRigidBody: adding restraints.

Applying restraints, which return the force and moment to be applied
to the attachment point.

Adding linearSpring with along axis damping.

Making access functions for the sixDoFRigidBodyMotion data private
where possible to prevent erroneous external use of body local data.

Fixing bug in unused updateForce(pointField...) function, torques were
not being correctly applied.

Adding gravity lookup to uncoupledSixDoFRigidBodyDisplacement BC to
allow it to be used in, for example, moveDynamicMesh (which would need
read gravity into the database).