Gazebo model - center of mass correction and complete inertia matrix

[gemenerik]

• Gazebo automatically places the center of mass at the origin of the model. This meant that the CoM has been at the wrong location. Fixed by shifting cf2_assembly.dae origin to center of mass. Should update Webots accordingly.
• First step at improving the Gazebo model accuracy by completing the inertia matrix with more values from Förster sysid.