added Null-Space Damping & removed Pi/-Pi Jumps for unlimited rotation joints

[sophielueth]

• Added Null-Space damping to the Task Impedance Controller, which improves stability in most joint configurations (only relevant for the 7dof version). The damping parameters can be tuned via dynamic reconfigure.
• Removed Pi/-Pi jumps for joints with unlimited rotation to Joint Impedance: Before, when you changed the desired joint position from +pi to -pi, the joint would rotate one full turn instead of not at all. Now, changes in the desired joint position will result in a motion according to the shortest path between the two joint positions, i.e., a change from +pi to -pi would not move the joint at all.