Automatically look for cross_sections.xml if path is directory

[ybadr16]

Description

Added the option to check automatically for "cross_sections.xml" if the user accidentaly set path to a folder instead of the file.

closes #3048

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)

Notes:
1- I had previously done this in #3054, however i didnt notice file_utils at all so i redefined headers and a new function in that pr. This one utilizes the already defined functions.
2- #3042 is moving to std::filesystem, but I dont think this would change anything in this commit since the functions remain defined in file_utils