AtomArrayStack with varying annotations per model

[entropybit]

Based on comment from @padix-key on #487 I opened this issue. The problem is that currently it is not possible to manage a collection of models with different annotations on a per model basis via AtomArrayStacks. Features like occupancy or b_factors and even charges could however vary for different models of even the same molecule.
This is a conceptual problem with the current layout of AtomArrayStack.
@padix-key correct me if any of this is wrong please.

[padix-key]

Yes, this is correct, currently the only annotation, that may differ between models is coord. Probably the most work needs to go into refactoring the file interfaces, the necessary changes to _AtomArrayBase should be quite small.