How do we add a new scalar to a pointcloud in python

### 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

Simple question.
How do you add and save a new scalar/channel to an point cloud using o3d in python? I have xyz - rgb point cloud where I would like to add other scalar (temperature), but not sure how to do it.

Thanks!