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Describe problem solved by this pull request
Implementation of a switch mode that allow the tilt of the rotors. For multi-copter models the Manual mode and the Stabilized one are equivalent so I decided to allow the tilt of the rotor for the Stabilized mode and disable for the manual mode. Another mode called "Tilt" was also created but it's only available in the terminal with the command "commander mode Tilt".

Describe your solution

• Create new mode as describe here : https://uav-lab.org/2016/11/07/px4-research-log-10-adding-a-new-flight-mode-preparation/
• Add some conditions in the tiltrotor.cpp and mc_attitude_control.cpp files

Describe possible alternatives
An alternative - more difficult - would have been to change the code of the QGround application to show the "Tilt" mode.

Test data / coverage
tested on Gazebo, works well