Cleaning up cob_gazebo_objects

[fmessmer]

I found several issues with the object models in cob_gazebo_objects, e.g.

• wrong xml-namespaces
• incorrect inertia
• unecessary links
• ...
  There are probably also several files formats which no longer can be spawned and such.

However, there are too many objects around to start fixing the issues for all objects currently present.
Thus, I suggest to first decide which objects are no longer needed.
Also, it needs to be tested whether the object can still be used as intended with ROS Indigo Igloo.
Finally, the required fixes and adaptions need to be applied.

@ipa-jsf and @ipa-rmb please contact @ipa-fxm in order to discuss this!

[fmessmer]

add all objects to urdf_object.test (ToDo from #106)