Implement cell importance variance reduction scheme.

[GuySten]

Description

This PR add cell importance to OpenMC as a simple variance reduction scheme.
Cell importance is ignored in problems with weight windows ( except 0 importance ).
When cell importance is used, global russian roulette is disabled.

Fixes #3063

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)