Adding receding horizon planning and trajectory optimization.

[malintha]

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[malintha]

This adds the following changes to the master branch:

1. Independent nodes for each drone (Add indepedent node control #4).
2. Add trajectory optimization support with ethz-asl/mav_trajectory_generation
3. Ability to publish goals for each drone separately. (Can we give navigation goal? #1 (comment))
4. A complete restructuring by including simulator_utils and geo_controller into the multi_uav_simulator repo, which previously needed to be cloned manually.
5. Use wstool to clone all the dependencies from ethz-asl/mav_trajectory_generation into the workspace.