Add Planar joint

[baggepinnen]

I must experiment a bit with state_priority to have the correct variables chosen, I have the following equations

       s_x ~ prismatic_x.s
        s_y ~ prismatic_y.s
        phi ~ revolute.phi

and despite state_priority being higher for s_x than for prismatic_x.s, I still get prismatic_x.s in the state