More improvements for the quadruped gaits example

These includes few improvements in this example, i.e.:

1. Added subplots for joint velocities and torques.
2. Added plot for CoM position.
3. Combined all phases in one single plot.
4. Removed DDP convergence plot since it's not interesting.
5. Used the geometrical center of the support region for defining the reference CoM positions.

The point 5 is needed to decoupled the results from one DDP solution to next one.