Add length_multiplier to scale DAGMCUniverse

[ssgudala-bwxt]

Description

Adds support for a length_multiplier argument on openmc.DAGMCUniverse so DAGMC geometry can be uniformly scaled at load time.

This change wires the new parameter through the Python API, XML/HDF5 serialization, and the C++ DAGMC initialization path. When provided, OpenMC scales DAGMC vertex coordinates before building.
Python-side bounding-box helpers are also updated to reflect the scaled geometry.

Fixes # 3200

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 added some small suggestions.

[ssgudala-bwxt]

Added your changes @GuySten

[GuySten]

Looks good to me except for two things.

1. I want to make sure that this PR does not affect performance of very large dagmc models.
   @shimwell, can you check if using this PR with length_multiplier for a complicated dagmc model does not affect loading time too harshly.

2. I think we should test this feature directly. For example by using stochastic volume calculation of a dagmc model of a ball with length_multiplier and checking it against the expected analytical result.

[ssgudala-bwxt]

Sure I can do #(2) @GuySten, what would be the best way for me to share that here? Just show the input and results?

[GuySten]

@ssgudala-bwxt, I meant adding a unit test.

[pshriwise]

Great idea @ssgudala-bwxt! I'll have a look at this in the next day or so.

[pshriwise]

Just one small ask for an HDF5 roundtrip unit test as well but then I think this is good to go.

Thank you @ssgudala-bwxt!

[ssgudala-bwxt]

I added two hdf5 unit tests. Let me know if one or both of them are what you had in mind @pshriwise

[pshriwise]

Thanks @ssgudala-bwxt!