Fix for plotting model with multi-group cross sections

[GuySten]

Description

openmc crash when trying to plot a model with multigroup cross sections.
This PR fix that.

Fixes #3747

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 @GuySten. I ended up removing all the extra code related to cross_sections. The recommended way to handle the data is to set openmc.config['mg_cross_sections'] before calling the plot() method. I'll comment on the original issue too.

[GuySten]

There were a few problems:

1. You plotted a surface so the material lookup did not happen.
2. When the mg_cross_sections are used as a relative path they do not work because when plotting we switch to a temporary directory.
3. I also added an informative error to the users to set mg cross sections path before plotting.

[paulromano]

Sorry about the mishaps in my updates. I did make another change to simplify the additions to Model.plot. The reason that the relative path didn't work in the test is because for testing we set openmc.config['resolve_paths'] = False, so the relative path is kept. However, the default configuration for users is openmc.config['resolve_paths'] = True which means that it should work out of the box for users. Instead of introducing config.patch in Model.plot, I changed it so that we set the absolute path in the test. Let me know if you're OK with this now and if so, we're good to merge.

[GuySten]

Now we are good to merge.