Add range sensor example

[srmainwaring]

Feature request

PR #5 provides support for range / proximity sensor integration in the core ArduPilotPlugin, but refers to an example in an external repo.

It would be good to have a simple rover example using the range sensors included here to help users get started.

Describe alternatives you've considered

Continue to reference external repos for examples. Not ideal as not self contained.

Platform
[ ] All
[ ] AntennaTracker
[ x ] Copter
[ x ] Plane
[ x ] Rover
[ ] Submarine

Additional context

The example referred to is here: https://github.com/srmainwaring/ros_gz_rover

[8-DK]

I have done this modifications on this branch.
https://github.com/8-DK/ardupilot_gazebo/tree/FR-40-rangefinder-sensor-integration
Please review code, if its ok to merge.

• Added special world and model file for lidar sensors. SDF file.
• Currently added 3(forward, rear and down) range finder in SDF. user can add upto 6 range finder.
• Changes in ArduPilotPlugin to get sensors topic similar to IMU sensor.

Use following params
RNGFND1_TYPE 100
RNGFND1_ORIENT 0
RNGFND2_TYPE 100
RNGFND2_ORIENT 4
RNGFND3_TYPE 100
RNGFND3_ORIENT 25