added contains() and remove() methods
added VoxelNode::inFrustum() which returns OUTSIDE, INTERSECT, INSIDE to allow more optimal frustum analysis
Optimized removeOutOfView()
- don't recurse down trees that are fully INSIDE of the view frustum, because those nodes
  are guaranteed to also be INSIDE
- changes to extraData of the recurse operation to make this possible
- adding debug stats to track how many nodes are inside, outside, intersect
- change call to removeOutOfView() to not call if last elapsed time has not
  yet passed since last call (similar to treeToArrays() logic)