Gazebo: Add model for OmniCopter

[srmainwaring]

This draft PR adds a Gazebo model for @IamPete1's OmniCopter

Usage

Gazebo and the plugins should be installed as per the ardupilot_gazebo wiki instructions.

Run Gazebo

gz sim -v4 -r omnicopter_runway.sdf

Run ArduPilot SITL

Copy the script Motors_6DoF.lua to the SITL scripts directory, then start SITL,

sim_vehicle.py -v ArduCopter -f JSON --add-param-file=$HOME/SITL_Models/Gazebo/config/omnicopter.param --console --map

Issues

The copter is not tuned and the attitude is not controlled. In 6DoF mode you can takeoff and CIRCLE but the copter constantly tumbles.