Introducing navigation classes that estimate safety and do the "level locate"
operation using the voxel structure managed by FlatVoxelManager.

In the large daughter limit, this provides better scaling than just using
the estimators based on BVH structure, since "safety" and "locate"
are very local operations.

This is a very first version for demonstration purposes.

Future directions/work will involve:
  a) automatic tuning of the right voxel size/structure
  b) combination of voxel structure with SIMD BVH
  c) speeding up creation of voxel structure
     (it is multi-threaded but we can make use of SIMD and GPU)

Adapt benchmark for SafetyEstimator accordingly.