New force elements. RigidBodyBuoyant allows for a center of buoyancy to be defined with regard to the body origin (not need to be at center of mass). Pulls information from a URDF file. RigidBodyAddedMass adds a force proportional to the acceleration of the body - important for underwater applications. This is achieved through a modification of the spatial inertia matrix of the parent body. Also pulls information from URDF file. New function in RigidBody allows for this modification without affecting the true mass of the body. RigidBodyManipulator edited to include the new force element definitions