Inverted Pendulum Actuated by Tendons

[DarioUrbina]

This is an example of PID control of an inverted pendulum actuated by strings. The pendulum axis is located on a cart that can only move pulled by two strings attached to it.

To create the bodies of this simulation, Issue #2922 was solved. You will see how I use a chain of bodies connected in a "loop" fashion.

When you run inverted_pendulum_tendon_actuation.py you will see how first the pendulum stabilizes, then it will move towards the white base. You can hit the pendulum close to its tip to change the displacement direction of the cart. If you hit harder it will try to stabilize.

As any PID system, depending on the gains, it will be more or less stable.

[erwincoumans]

Thanks for the PR, that looks like an interesting example. Some change in constraint.py slipped in the PR, we can remove it, right?