Append voxels to VoxelGrid

**Describe the bug:**
I tried to append Voxels to the VoxelGrid. There was one related issue #1101. I tried it the same way but I get the error:
```
open3d.cpu.pybind.geometry.VoxelGrid' object has no attribute 'voxels'
```
There seems to be some functionality within the C++ class:
https://github.com/intel-isl/Open3D/blob/49d7fc2b88b8cee4ad733f1c439eb12e49955472/cpp/open3d/geometry/VoxelGrid.h#L130

What is the recommended way to add own Voxels to a given VoxelGrid in python?

**To Reproduce:**
```
vg = o3d.geometry.VoxelGrid()
voxels = []
vox = o3d.geometry.Voxel()
vox.grid_index = np.array([ 7, 59, 101])
voxels.append(vox)
vg.voxels = voxels

```

**Environment (please complete the following information):**

- OS: Ubuntu20.04
- Python version: 3.8.5
- Open3D version: 0.11.2
- Is this remote workstation?: no
- How did you install Open3D?: pip