Stacker drops non-ENZ components

[koepflma]

Hi noisepy-team,

I'm working with a nodal dataset where we have the components ??1, ??2, ??Z. When I wanted to stack the components, all pairs except of ZZ were dropped which might be the case because the other pairs do not fit enz_system = ["EE", "EN", "EZ", "NE", "NN", "NZ", "ZE", "ZN", "ZZ"] in stack.py function stack_pair. It would be great if the code could also handle different component notations.

Thanks!
Manuela