Subcritical Multiplication Run Mode

[mlouis9]

Description

Implements this method for subcritical multiplication (fixed source) simulations as a new run mode. Currently, this method is identical to eigenvalue mode except that instead of purely sampling from the fission bank, it samples from the external source with probability (1-k), resulting in convergence to the fixed source solution rather than the fundamental mode, and hence $k \neq k_{\text{eff}}$. Currently, tallies have to be manually normalized by $M = \frac{1}{1-k}$ to be comparable with fixed source mode. Future plans to integrate this feature with #3788 so $k_q$, $k_s$ can be directly calculated and plans to automate normalization by $M$.

Note portions of the documentation are identical to #3788.

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)