Add a length multiplier option for XDG meshes.

[nuclearkevin]

Description

I've been working on integrating XDG mesh tallies into Cardinal, and found that the "external XDG" option was missing an optional parameter for length scaling (which we use to match OpenMC models in cm to MOOSE models in m).

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)

[paulromano]

Thanks @nuclearkevin!