Visualize 3D contact forces in the Mujoco passive viewer

[diegoferigo]

PR #83 introduced initial support of visualizing the kinematics of simulated models with the passive Mujoco viewer. It would be pretty interesting to exploit the same viewer to also visualize the 3D contact forces applied to all collidable points.

[diegoferigo]

For those that might be interested in adding support for contact forces visualization, I'd start exploring the programmatic addition of arrows using the user_scn attribute of the viewer together with:

• mjv_initGeom
• mjtGeom