Allow spawning multiple prefabs

[francofusco]

This PR is closely related to #77 and uzh-rpg/flightmare_unity#8. The goal is to improve and show how to insert and move/scale different static objects in Unity. As mentioned in uzh-rpg/flightmare_unity#8, it might be interesting to allow not only to add but also remove objects at runtime. If you wish such a feature, please do not merge this PR yet :wink:

• Commit be21a6d:
  • Fixed missing macro in StaticObject - my bad, I forgot it in my previous PR #107 :sweat_smile:
  • Added the implementation of Quadrotor::getQuaternion() (it was declared in the header, but the implementation in the cpp had likely been deleted by mistake or forgotten)
• Commit 0bcc5a1 simply allows to update the scale of quadrotors and objects from the client side. If I am not wrong, the feature was already available on the server (Unity) side. I am not sure if this feature has any practical usage - perhaps simulating an inflating blimp? I can remove the feature if you believe it should not be here.
• Commit 366e071 finally adds an executable in flightros that can spawn and move arbitrary objects in the scene. Just a demo to show that the sibling PR uzh-rpg/flightmare_unity#8 works!

[francofusco]

Ping!

[slimeth]

@francofusco LGTM Adding the feature to remove objects would be a great extension :+1: