ROS: modify iris example to support sdformat_urdf

[srmainwaring]

Modifications to support running ArduPilot SITL / Gazebo with ROS 2.

The ros2 branch includes changes to make the iris model compatible with the package sdformat_urdf. The package allows a SDFormat robot model to be used as a robot description by ROS tools such as RViz, however it imposes some restrictions on the model.

Details

Changes to model structure

• The prefix model://{model_name} must be replaced by package://{package_name}/models/{model_name}.
• Nested models must use the //model/include/@merge="true" attribute which requires elements in included models to have unique names.

Transform fix

• An additional rotation is needed to correctly map from the Gazebo ENU world frame to the ArduPilot NED frame.

Rendering

• The rotor blades are colour coded to match the ArduPilot wiki frame diagrams (Blue => CCW, Green => CW)

Additional sensors

• The model has additional sensors to illustrate Gazebo sensor capabilities and bridging the topics from Gazebo to ROS.

Future work: merging changes back into main

• The proposed change Handle model:// path in URI ros/sdformat_urdf#20 should address the need for the package prefix.
• The changes to the behaviour of nested models are governed by http://sdformat.org/tutorials?tut=composition_merge_proposal which imposes some restrictions on naming model elements.