adding trigger to mgxs convert

[shimwell]

Description

At Physor 2026 an chatting with @jtramm and @pshriwise about making generation of multigroup use a trigger to allow it to end earlier if the reaction rates all meet the trigger relative error value. At the time it sounded like a good idea but in practice it appears to have enlarged the api interface to users which I was trying to avoid. I wanted to make this a draft PR just to show @jtramm and @pshriwise that I tried.

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• 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)