Adds MPACT library group structures and documentation to mgxs collection

[bscollin]

Description

Group structures from the MPACT reactor physics code are added to the mgxs.GROUP_STRUCTURES python dictionary. These include a 51g structure used primarily for PWRs, a 60g structure developed for BWRs, a 69g structure developed for Magnox reactors, and a 252g structure that should be good for most thermal systems.

Fixes # N/A

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)

[paulromano]

Thanks for this contribution @bscollin! I looked at the reference provided in the comment but I only see the definition for the 51 group structure there. Where can one find the definition for the other structures?

[bscollin]

@paulromano, I pulled the data straight from the cross-section library files distributed with VERA. I talked with Aaron Graham @aarograh, the VERA project lead at ORNL and he was okay with releasing these. I did find a reference to the 69g Magnox library: https://www.epj-conferences.org/articles/epjconf/pdf/2021/01/epjconf_physor2020_10016.pdf
and the 252g is the same as scale:
https://serpent.vtt.fi/mediawiki/index.php/SCALE_252-group_structure
It doesn't look like they have published the BWR library structure anywhere. I can back that one out if you would like me to. I can also add the extra references, I just didn't want to flood the references with MPACT ones.

[paulromano]

Thanks @bscollin. Could you put those references in the docstring below GROUP_STRUCTURES? Also, should we refer to the 252 group structure as "SCALE-252" instead of "MPACT-252"?

[bscollin]

@paulromano All requests addressed. Added references to 69g library, renamed 252g library to SCALE-252 and added reference.

[paulromano]

Thanks @bscollin!