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.
By switching from set to list you give up the invariant that there are no duplicate entries. Just to make sure: was this assumption used in code? And if so, do we now have checks to make sure it holds?