[GAZEBO_9] Add Mellinger's and Silano et. al controllers

[gsilano]

The new version aims to add two news controller algorithms: the Mellinger [1] and Silano et. al [2]. This branch concerns the Gazebo 9 version.

[1] D. Mellinger and V. Kumar, "Minimum snap trajectory generation and control for quadrotors," 2011 IEEE International Conference on Robotics and Automation, Shanghai, 2011, pp. 2520-2525. DOI: 10.1109/ICRA.2011.5980409
[2] G. Silano, P. Oppido and L. Iannelli, "Software-in-the-loop simulation for improving flight control system design: a quadrotor case study," 2019 IEEE International Conference on Systems, Man and Cybernetics (SMC), Bari, Italy, 2019, pp. 466-471. DOI: 10.1109/SMC.2019.8914154

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gsilano]

Hey, bot! I'm still working on this. Remove the wontfix tag.

[gsilano]

Although the pull request is not complete (the Mellinger controller still does not work), I still thought about merging the changes into the master. New features, such as internal model control and a spline trajectory generator, have been provided to the repository. I think these are useful, even out of the Crazyflie context. I will continue to work on the Mellinger controller to also provide this functionality to the ROS repository, but without a stringent deadline.
In README.md, I will put a note on this.