[fmrico]

Closes #168.

New package easynav_bonxai_planner (plugin easynav_bonxai_planner/BonxaiPlanner). It plans 3D paths for aerial robots over map.bonxai, as follows:

• Clearance grid: an exact Euclidean distance transform of the occupied voxels, built once per map.
  • fill_below sets what counts as solid below the scanned surfaces. lowest makes the ground solid. highest makes buildings solid blocks, as in an elevation map.
  • Columns with gaps take their neighbours' height.
• Search: 26-connected A* that only goes through cells at least min_clearance from the obstacles. Steps closer than preferred_clearance cost more.
• Start and goal: if one is too close to an obstacle, it moves to the nearest free cell within snap_radius.
• Path: shortcuts wherever the clearance allows, then sampled every path_step and smoothed. The smoothing never moves a point closer than min_clearance.
• No path: the warning says why (start blocked, goal blocked, unreachable).

Tests: unit tests for the distance transform, fill_below, A*, snapping and the smoother, plus plugin tests.

In Gazebo: with the MPC from #169, it guided the UFO of the aerial playground through a town, between buildings and over them.

Builds: Humble, Jazzy, Kilted, Lyrical and Rolling.