TSDF extract_voxel_grid gives color€[0,1] and not signed distance

### 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

How can we know from colour value whether particular voxel is outside the surface, on the surface or inside the surface ?

Signed distance could have helped. Any way we can get the signed distance of every voxel from the nearest surface ??