Adding possibility for a collection of structures

[gezmi]

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

[gezmi]

In the same notebook, I have also implemented superimposition, using TMalign