Added const modifiers to OctreePointCloud function

- Added const keyword to function 'OctreePointCloud::getVoxelBounds ()',
  since it internally only calls a 'const' function
  ('OctreePointCloud::genVoxelBoundsFromOctreeKey ()')
- Added const modifier to 'OctreeIterator' input argument of
  'OctreePointCloud::getVoxelBounds ()', since only 'const' functions
  of 'OctreeIterator' are called