Compatibility with Voxgraph based Planning

[gasserl]

The goal of this PR is to enable planning based on a voxgraph map. The PR contains a collection of changes to allow reusing existing planning methods and code.

Changes:

• Separating the RRT planner into a parent and chlid class to be reused for planning based on voxblox as well as voxgraph
• Introducing a map interface to separate the mapping from the planning for the RRT planner. The interface and specialized voxblox class are in a separate package named "mav_planning_voxblox"
• Minor adaptions to skeleton graph to allow for local submap graphs
• Introducing a verbosity attribute in the skeleton planner and generator

[jaiswalharsh]

Hi Laura,

Is it possible to test this with voxgraph already? It would be great to know if you can point to some other planners that could work with voxgraph based mapping.

Thanks in advance.