Support Mixture distributions in combine_distributions

[GuySten]

Description

Currently combine_distributions does not support Mixture distributions.
Support of Mixture distributions is important for applications (like r2s) that involve some burnup chains.
This PR add support for Mixture distributions to combine_distributions.

Fixes #3445 (comment)

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 @GuySten!