Fix IFP implementation

[JoffreyDorville]

Description

The IFP (Iterated Fission Probability) method is used to calculate effective kinetics parameters in OpenMC. An error was discovered in the way the precursor group numbers were stored. With this correction, the precursor group of the neutron initiating the fission is effectively stored (previously it was the precursor group of the neutron created from this fission event).

This modification only impacts the results obtained for the beta effective.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) 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)

[paulromano]

Thanks for the fix @JoffreyDorville!