Adding possibility for a collection of structures

#### Describe the workflow you want to enable
I would like to be able to load structures from different sources, into a collection (stack) and make it easier to apply different functions (calculations, filtering) for all the structures at the same time.

#### Describe your proposed solution
I have implemented a stack solution here, will move it to be properly part of the package if approved.
https://colab.research.google.com/drive/1a54jnLzzNUYoLBcwweHdbrjIqEKUJtVP#scrollTo=lOspp7m91ve4

Start by reviewing the linked Google Colab, which contains the proposed stack implementation; the issue does not name a package file, entry point, or test. Done would mean an approved collection API can load structures from different sources and apply calculations or filtering across them, with the relevant behavior covered by tests.

Written by the indexing model from the issue text.