Make OsgOctreeRepresentation serializable

OsgOctreeRepresentation previously accepts an OctreeShape and
retrieve the Octree stored in the OctreeShape and visualize it.
However, this is not serializable since the OctreeShape is not stored.

After this change, OsgOctreeRepresentation will store a shared reference
to the OctreeShape. This OctreeShape will be serialized and deserialized
during serialization.