Voxel explosions

Adds VoxelExplosions spawned by Bullets and Rockets upon impact. Performance is moderate in release and unplayable in debug.
- Uses collisionFiltering, Explosion particles don't collide with each other nor with bullets
- Allows an impactVector to be passed to the generator so the explosion is not always a sphere
  - currently not used
  - awesome for debris later on
- Press B (for boom) to spawn a random explosion in front of you
- Gives the specialvoxels more hp in .ini #170
- Please feel free to play around with the vfx config variables