How to interpret compute_point_cloud_mean_and_covariance output

### 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

I see that it outputs 2 tuples.
1st one a 3 dimensional and 2nd being 3*3 . i wanna understand layman, english sense for these numbers.

If i understand right, the 3*3 is covariance matrix and eigen vector to it will give principle directions.

If some one could help me on how to make sense of it, it would be great