Generate Region Plots Directly

[pshriwise]

Description

A continuation of #2648, these changes allow regions to be plotted directly without need for a cell definition. A simple region can now be plotted with the following:

import openmc
from matplotlib import pyplot as plt

sphere = openmc.Sphere()
x_plant = openmc.XPlane()
hemisphere = -sphere & +x_plane
hemisphere.plot()
plt.show()

There was a little more nuance here as the color_by, color, legend, and legend_kwargs arguments no longer have meaning at this level of geometry definition, so I've addressed those as best I could for now.

Other

Also cleaned up the openmc.Cell.plot docstring a bit.

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)

[shimwell]

Nice PR. We might be able to plot everything thing soon 🚀
Region - this PR
Cell - done
Geometry - done
Universe - done
Model - PR #2863

[paulromano]

Thanks!

[shimwell]

Looks like that extra assert in the testing picked up an error in the latest CI run.

 >       assert c_after.id + 1 == c_before.id
E       assert (2 + 1) == 1