Fix for issue #905. 

Fix for problem (issue #905) where leaves would not be found during seeding, and warning would show up - now we use leaf indices directly for seeding, instead of doing a reverse look up. This is faster and fixes the problem.

Changed OctreePointcloud Adjacency to use std::list instead of std::set.
Added comparator to SupervoxelHelper so that the internal set of voxels owned by a helper is sorted by index instead of memory location. 

Supervoxels and OctreePointcloudAdjacency are now fully deterministic.
I think I'll rename that typedef too...