Allow StepResult.get_material to accept integer material ID

[AlvaroCubi]

Description

The StepResult.get_material method uses str for mat_id. Usually we pass int as ids. Now the function has been updated to work both with strings and integers.

A test has been added.

Fixes #3869

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)