Expose universe plotting for the geometry class

[shimwell]

As mention in #2648 it would save users accessing the root_universe when plotting a geometry if the plot method was exposed to the openmc.Geometry class.

Checklist

• I have performed a self-review of my own code
• I have run clang-format 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]

Nice, @shimwell! Pretty straightforward, but I'll give it a 24 hr cooldown for others to take a look as well.

[pshriwise]

I went ahead and ticked the testing box for you @shimwell. Given that this relies on the already-tested Universe.plot method I don't think it's necessary to include additional tests here.