add export_model_xml arguments to Model.plot_geometry and Model.calculate_volumes

[nplinden]

Description

This adds an export_model_xml argument to Model.plot_geometry and Model.calculate_volumes, for coherence with the Model.run method.

The implementation mimics that of Model.run and the new argument defaults to True.

Previously running these methods in the same script resulted in populating the working directory with model.xml as well as the geometry/materials/settings/plots/tallies.xml, making things ambiguous.

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)

[paulromano]

Thanks @nplinden! I made one small update to ensure that if you pass path='other_name.xml', it gets passed to plot_geomtery and calculate_volumes accordingly.