IMPORTANT: Please use the following template to report the bug.

Describe the bug
open3d.geometry.VoxelGrid.voxels.append(voxel) does not append the voxel object or any other object.

My goal is to create a VoxelGrid object from a numpy array of shape (128,128,128).

To Reproduce

import open3d as o3d
import numpy as np

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

but print(vg.voxels) just returns []
and of course print(vg.has_voxels) says <bound method PyCapsule.has_voxels of geometry::VoxelGrid with 0 voxels.>

Expected behavior
vg.voxels should contain the Voxel object

Environment (please complete the following information):

• OS: OSX Mojave 10.14.5
• Python version: 3.6.5
• Open3D version: 0.7.0.0
• Is this remote workstation?: no
• How did you install Open3D?: conda