Resolve relative input paths relative to XML files

[paulromano]

Description

Currently, if you call Model.from_model_xml() on a model.xml file that is not in your current directory, any files that it references with a relative path will be treated as though they are relative to your current working directory rather than the parent directory of the XML file itself. This PR fixes this so that XML-loading entry points now establish the directory containing the source XML file as the path-resolution context. Nested inputs such as DAGMC geometry, meshes, and other referenced files are therefore found relative to the XML file rather than the process's current working directory when openmc.config['resolve_paths'] is enabled. The context is scoped and safely restored after parsing, including nested XML-loading operations.

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)

[GuySten]

I've updated plots.py.
Looks good to me now.