Missing belt ratio in revolute pair with rotors joint

[MatthewChignoli]

From Andrew

Hey Matt, no rush on this but I'm having a bit of trouble re-deriving the rotor angles in the grbda code for the revolute pair joint...it seems like the revolute pair joint without rotors uses the absolute angles as the generalized coordinates, but the revolute pair with rotors does use the relative angles as the generalized coordinates. however, I think there should be three belt ratio parameters, rather than two, since there are three belts in total, and all of them could have different ratios (it just so happens that on the humanoid and the arm the ones in the same link have the same ratio)