[DO NOT MERGE] Example of how to swap the backend from octomap to voxblox

[helenol]

Please don't merge this, just for reference for those wishing to switch NBVP from octomap to voxblox.

This is a very simple wrapper that just mimics the interface to octomap to voxblox. It was made to have the behavior as close as possible.
As such:

• It only uses TSDFs, not ESDFs for collision checking. Using ESDFs would require pre-computing the ESDF with an ESDF server, and using spheres as the collision checking model. However, it would be significantly faster.
• The bounding box line check is done very naively (read: slowly), repeating a lot of the operations. This could be re-done easily in a more intelligent way.
• The way it's currently set up, the nbvp planner both does the planning and builds the map (before octomap, now voxblox map). This is not recommended. The recommended set-up is to have a separate node building the map and passing it to the planners, as in the mav_voxblox_planning gazebo examples (here)

This is by no means a great implementation, just a sample to get people started. :) Please see mav_voxblox_planning for many more planning examples.