Fix for visualising attached shapes

[VladimirIvan]

Shapes created by defining custom links do not get published when calling publish_frames. This was because they were considered robot as robot geometry and got ignored. This avoids displaying the robot geometry twice, 1st as a part of the robot model and 2nd as the collision object published as a MarkerArray.

• Adds a separate logic to publish custom shapes attached to the robot but ignoring the shapes defined in the URDF.
• The links defined in the URDF are now correctly marked with is_robot_link=true. This is then used to distinguish links loaded from the URDF from custom links.