fix the bug in function differentiate_mats()

[cn-skywalker]

Description

I found that differentiate_mats method cannot assign the distributed material properly. When I use the ‘match cell’ method, only one material rather than a list is assigned to the repeating cell. That’s seem to be a bug.
Now it has been fix in this pr.

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)