avoid need to set particles and batches for dagmc models when calling convert_to_multigroup

[shimwell]

Description

If we call convert_to_multigroup with a dagmc model currently and the settings have no particles or batches then it fails validation.

IMO we shouldn't need to set particles or batches before calling convert_to_multigroup as convert_to_multigroup has an argument for nparticles and decides internally the batches particles split.

This PR just sets the batches and particles to a valid, this will help it pass validation but it does not actually get used in simulation

If we ask user to set particles and batches on the model.settings then this could be a bit confusing as they are not used by convert_to_multigroup

What do people think is this an acceptable hack?

Fixes # (issue)

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)

[pshriwise]

One alternative might be to initialize OpenMC in a non-transport mode when synchronizing DAGMC universes

[shimwell]

Thanks for the suggestion @pshriwise I've had a go at implementing the transport free option and also added a test.

[pshriwise]

Tiny suggestion here but otherwise looks good @shimwell!

[jon-proximafusion]

note to self CI is failing with

E                   TypeError: Model.init_lib() got an unexpected keyword argument 'args'

[pshriwise]

note to self CI is failing with

E                   TypeError: Model.init_lib() got an unexpected keyword argument 'args'

Hrmmm yeah it seems we allow only a subset of command line arguments to go through to the openmc.lib.init call. Would probably be better if we simply accepted an cl_rags keyword dictionary for flexibility in hindsight.

[shimwell]

Thanks for fixing this @GuySten and in particular getting the CI passing
Are you able to take another peak at this @pshriwise

[pshriwise]

Looks good @shimwell! Thanks!