How to access to octree voxel coordinates?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hi,

I'm working with SPH (smoothed particle hydrodynamics) and need to reconstruct the surface of a simulated fluid. The built-in methods for surface reconstruction in Open3D does not work on my case as I have a point cloud where points are sampled inside an object not just on the surface. So, I'm trying to reconstruct the surface using **marching cube**. To do this, I need to voxelize my space and Octree is a optimzed data structure for this aim.

I have made octree for my point cloud using Open3d. I'm wondering how can I access to octree voxels' coordinats as I need them to compute a value in each point and apply marching cube on them.