moving gz into submodule

[frede791]

Striving for greater autonomy between PX4 and Gazebo, this PR moves gazebo models and worlds into their own repository and adds them as a submodule in PX4. This is another step in separating PX4 and Gazebo. Furthermore, this PR moves the tools folder from the /simulation/gz/ folder to the /simulation folder in order to not need it to import it as well.

Solution

Moving gz models and worlds to their own submodule for greater autonomy.

[frede791]

@dagar Let me know what you think of this. This is another step in moving Gazebo models away from PX4.
There are still some cmake warnings being thrown being building so I will work on fixing those before marking this PR as ready for review.

[DronecodeBot]

This pull request has been mentioned on Discussion Forum for PX4, Pixhawk, QGroundControl, MAVSDK, MAVLink. There might be relevant details there:

https://discuss.px4.io/t/px4-maintainers-call-november-28-2023/35555/1

[dagar]

Can you have cmake add the submodule so that it doesn't need to be manually initialized?

PX4-Autopilot/src/modules/simulation/simulator_mavlink/sitl_targets_flightgear.cmake

Line 7 in a9213e3

px4_add_git_submodule(TARGET git_flightgear_bridge PATH "${PX4_SOURCE_DIR}/Tools/simulation/flightgear/flightgear_bridge")

Place it here so that it's only initialized if gz_bridge is in the build.

https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/simulation/gz_bridge/CMakeLists.txt

[frede791]

@dagar amended Cmake to auto initialize the gz submodule

[frede791]

has been made redundant by #22467 and can therefore be closed.