Fix a bug in openmc.data.combine_distributions

[GuySten]

Description

This pr change the implementation of combine_distributions to support combination of any Univariate distributions with specified probabilities.

The new implementation knows to combine analytically discrete and normal distributions.

Fixes #3105

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)
• [x] 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)