Add glider fixedwing model

[Jaeyoung-Lim]

Problem Description
This commit adds a unpowered fixedwing model. This model can be useful for not only developing gliders, but also observing behaviors in TECS.

The model is a modified version of the standard plane model. The rotors are removed from the vehicle.

In order to takeoff, the simulation is equipped with a catapult_plugin, which throws the vehicle to up to around 30m altitude.

Testing
To run SITL, run the following command.

make px4_sitl gazebo_glider

The vehicle takeoff can be triggered by typing

commander takeoff

in the mavlink shell

[LorenzMeier]

It looks good - have you seen the failed check?

[Jaeyoung-Lim]

@LorenzMeier Yes, I think the failure is related to PX4/PX4-Autopilot#17644 where the latest tbb is causing issues for gazebo.

Do you have any ideas on how to fix this other than reverting tbb versions as suggested in the issue? Does this need fixing on the gazebo tap side?

[hamishwillee]

Hi @Jaeyoung-Lim I see no glider: http://docs.px4.io/master/en/simulation/gazebo_vehicles.html
Any intent to add? Any other new models added to codebase but not in docs?