Including cross_section.xml path in model.xml

[shimwell]

Description

currently setting the cross section either using
openmc.config or material.cross_sections or environmental variable does not result in the information on where to find the cross sections being written to the model.xml

So running a python script where cross section is set then running openmc executable on the model.xml and this error is caused.

ERROR: No cross_sections.xml file was specified in materials.xml or in the
        OPENMC_CROSS_SECTIONS environment variable. OpenMC needs such a file to
        identify where to find data libraries. Please consult the user's guide
        at https://docs.openmc.org/ for information on how to set up data
        libraries.

Perhaps when the cross section on the material object is set then this could also be written to the model.xml file and materials.xml

Then perhaps some other changes and openmc executable could find the cross sections.

Alternatives

Move the openmc.config to the settings object so that the cross section path is written out in the settings part of the xml

Compatibility

I think generally this would improve user experience when running from python or with the openmc executable.