[ros2] Minor updates for demos

[AndrejOrsula]

🦟 Bug fix

Summary

Few small changes, nothing significant

• Re-enable air pressure demo
  • Fixes Bridge FluidPressure for ROS 2 #78
• Add image_topic launch argument for image_bridge demo and use corresponding topics for depth and rgbd camera demos (instead of manually searching for the topic in drop-down menu)
• Add RQt topic viewer to IMU demo, so that people have a way to see the output until RViz2 has IMU topic visualisation
• Do not normalize depth image in RViz2 config and set its corresponding clip range. This allows it to be viewed (see comparison below - I am not sure about the glitch in the first few pixels though)

Before:

After:

Issues not resolved by this PR

The following issues still remain, but they all already have a TODO/FIXME note connected with them:

• All demos blocked by "Blocked by ros_ign_point_cloud issue"
  • Depth camera, RGBD camera, GPU lidar (they all have a TODO note connected with them)
• Battery
  • RQt is not plotting the data properly
  • There is a FIXME for this inside battery.launch.py script
• IMU
  • RViz2 still does not seem to have IMU topic visualisation

Checklist

• Signed all commits for DCO
• Updated documentation (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)

Note to maintainers: Remember to use Squash-Merge

[chapulina]

I'll close and open to nudge Travis

[chapulina]

🎉