Load .obj files as scenes instead of as a single model

### 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).

### Proposed new feature or change

It would be great that if an .obj file have multiple objects, instead of loading it as a single model in a [TriangleMesh](http://www.open3d.org/docs/0.9.0/python_api/open3d.geometry.TriangleMesh.html) it could be store in something like [TriangleMeshModel](http://www.open3d.org/docs/latest/python_api/open3d.visualization.rendering.TriangleMeshModel.html#open3d-visualization-rendering-trianglemeshmodel) or loaded independently. I've been searching in your library and couldn't find this feature, at least is not obvious how to use it if it exists already.

### References

In the Trimesh library they have it as a default feature of its loader, you could see it [here](https://stackoverflow.com/questions/59417881/select-specific-mesh-in-obj-file-or-edit-file)