Updates to publish robot model

[bponsler]

Simple changes to the robot state publisher to publish the robot model URDF as a std_msgs/String. This is to allow rviz, or other nodes, to access the robot model.

[bponsler]

Note that I decided to publish the robot model (instead of write it as a parameter) for a few reasons:

1. Publication support seems a little further along than parameters
2. Easier support for a (potential future) dynamic robot model. Other nodes will receive model changes and not need to query for changes to the parameters
3. Fastest way to get the robot model plugin working in rviz (probably the main reason, :) )

I also chose to duplicate the robot_model::initFile function so that I did not also have to make changes to two the robot_model repo. An improved solution would be to modify that function instead of having the separate copy (additionally, that would keep the file from being read twice).

[wjwwood]

lgtm

I made a few fix ups and improved one of the error messages.

Sorry @bponsler that it took so long to add this. I was going back and forth on whether or not to add this and finally when we implemented it in the ros2/rviz repository, we added support of topic or parameter, so I opted to go ahead and include this for now.

Thanks for the contribution and sorry it sat so long.

[wjwwood]

CI:

• Linux
• Linux-aarch64
• macOS
• Windows

[bponsler]

Awesome! Thanks for pushing it over the finish line - glad it ended up being useful.