message: add getter for voxel size inverse in layer
diff: @@ -151,6 +151,7 @@ class Block {
  // Basic function accessors.
  size_t voxels_per_side() const { return voxels_per_side_; }
  FloatingPoint voxel_size() const { return voxel_size_; }
  + FloatingPoint voxel_size_inv() const { return voxel_size_inv_; }
  size_t num_voxels() const { return num_voxels_; }
  Point origin() const { return origin_; }
  FloatingPoint block_size() const { return block_size_; }
  