using gnds and zam from endf-python

[shimwell]

Description

This PR removes some local functions and makes use of endf-python that includes those functions.

These functions are useful to users so important that they are still findable.

To ensure they are listed in the docs as openmc.data.* instead of endf.* I have added a reassign

note we made a small change and Paul kindly released a new version of python-endf to make this one possible
paulromano/endf-python#31 (comment)

Fixes # (issue)

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)
• I have added tests that prove my fix is effective or that my feature works (if applicable)