Calculation of kinetic parameters relevant to fissile system with an external neutron source

[GuySten]

Description

This PR extend OpenMC capabilities to calculate kinetic parameters relevant to fissile systems with an external neutron source (i.e. ADS, reactor startup).

The changes needed include calculating standard deviation of k of each generation and storing it in the statepoint in the cpp side and simple postprocessing on the python side.

Theory and Validation

The implementation and testing follows the following papers:

• The equivalent fundamental-mode source
• MCNP Calculations of Subcritical Fixed and Fission Multiplication Factors

Checklist

• [x] I have performed a self-review of my own code
• [x] I have run clang-format (version 15) on any C++ source files (if applicable)
• [x] I have followed the style guidelines for Python source files (if applicable)
• [ ] I have made corresponding changes to the documentation (if applicable)
• [x] I have added tests that prove my fix is effective or that my feature works (if applicable)