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.
@taketwo No, there are no checks at the moment to make sure no duplicate neighbors are added.
Neighbors are added quite simply - we just check the 26 neighborhood by octree keys and add them if they exist.

There is no way to add new points into the octree after it is constructed, so the only function that modifies the list of neighbors is computeNeighbors in octree_pointcloud_adjacency - meaning that the only way to get duplicates is if a user manually adds in neighbors. 

I'm thinking I should modify the OctreePointCloudAdjacencyContainer so that only const_iterators and getData() can be used from outside of OctreePointCloudAdjacency. It doesn't really make sense for users to be messing with anything else.