Get an iterator over a bounding box.

[OasisArtisan]

Firstly, I have cited OpenVDB in our latest publication in IROS called RayFronts and your library was very useful in doing efficient occupancy mapping so thanks for the great work.

I am trying to figure out the most efficient way to iterate over active voxels in a specific bounding box.
Naive solutions would be:

1. Iterate over all active voxels and check if they are in the bounding box
2. Use an accessor to check all coordinates of a bounding box if they are active voxels or not.

I think its safe to say there is probably a smarter/faster way of doing this where the iterator itself can quickly jump to where the active voxels in the bounding box is. Even if its rough and I still need to check if its within the bounding box or not but it would save time.

Your help is greatly appreciated.