twist convention for URDF links

[iamtesch]

Ensure the convention for the "twist" variable when creating an X5-link matches that of the rest of the API.

[iamtesch]

Confirmed that it does with the new xacro restructure

[ankitbhatia]

There is a bug in the origin computation for the visual and collision geometry generation for the output flange A-2039-02.stl.

The correct origin is
<origin xyz="${extension} ${-sin(twist) * edge_to_center} ${edge_to_center * (1 + cos(twist))}" rpy="${twist} 0 0"/>