Add VoxelTreeNode Pointer to Voxel, speeds up removal from CollisionDetector

(significantly) 

....also removeVoxel() now gets passed a pointer to a Voxel. In most cases the function was called like removeVoxel(voxel->gridCell()) anyway.

This doesn't fix #157 and #167 completly, but improves them, at least the damageForwardLoop, quite a bit.

Minor:
* add override to some virtual hooks
* add planet model to test deep objects