`FrameShape3DReadOnly#copy` should return `FrameShape3DBasics`

[Duncan Calvert]

I want to be able to copy a shape and then attach it to a different reference frame. That's pretty much the only way you can move it around, as the pose is usually some constant offset to the parent, which needs to be kept.

[Sylvain]

Hmmm yeah, I think I was facing some kind of issue when doing so but can't remember, I'll take a look