Stacks should share the geometry objects

[hamogu]

Elements in a stack should share the same geometry object. I've come across user code (written by me, but outside of MARXS) where I tweaked the geometry object of a stack, but the change did not propagate down to the objects that make up a stack.
I'm not sure how to solve this. It's pretty easy if I just want to work with updates of the existing geometry object. In that case, the objects in the stack can just point all to that object. However, if the geometry object that the stack owns is replaced wit ha new one, how do I update all the other references?

Related to #118 which suggests relative geometry expressions.