How to scale an entire group of nodes?

I used `let mut g = window.add_group()` to create a `SceneNode`. Next, I added a bunch of spheres to this group using `let mut obj = g.add_sphere(10.0)`. Then I moved these spheres to random locations with `obj.set_local_translation`.

Now, I want to scale the entire group with respect to the origin. But the only scale function in the group I see is `set_scale`; which only scales the spheres individually, and doesn't affect their positions at all. How do I scale the entire group with respect to the origin?

Start by tracing the SceneNode APIs mentioned in the issue: add_group, add_sphere, set_local_translation, and set_scale. Verify how group transforms affect child positions in a small scene; done means scaling the group also scales the spheres' positions relative to the group's origin.

Written by the indexing model from the issue text.