Support exporting closed-loop kinematics structures

[traversaro]

In some context it would be useful to be able to export closed-loop kinematics structures via creo2urdf . As a first example, being able to export a four bar linkage would be interesting.

[traversaro]

I am opening this issue after a meeting with @CarlottaSartore @xela-95 @rob-mau just to track the idea, and to link the related issue on the idyntree side: robotology/idyntree#750 .

[traversaro]

@Nicogene this was the issue that I was referring to yesterday. We probably could do some of the work described in robotology/idyntree#750 to minimize the effort (and debugging) required in creo2urdf.