adding optional outline to plots

[shimwell]

This PR attempts to add outlines to the universe.plot method.

I had this functionality over in the openmc-geometry-plot package and thought it would be much better over here ♻️

Also inspired by @gridley and @pshriwise recent PR which improves the universe plot method.

before this PR

after this PR using optional outline

Minimal script for using this outline feature

import openmc
from matplotlib import pyplot as plt

mat_iron = openmc.Material()
mat_iron.name= 'iron'
mat_iron.add_element("Ni", 1.0)
mat_iron.set_density("g/cm3", 1)

mat_aluminum = openmc.Material()
mat_aluminum.add_element("W", 1.0)
mat_aluminum.set_density("g/cm3", 19.3)

sphere_surf_1 = openmc.Sphere(r=20, boundary_type="vacuum")
sphere_surf_2 = openmc.Sphere(r=1, y0=7)
sphere_surf_3 = openmc.Sphere(r=5, x0=7)

sphere_cell_1 = openmc.Cell(region=-sphere_surf_1 & +sphere_surf_2 & +sphere_surf_3)
sphere_cell_2 = openmc.Cell(region=-sphere_surf_2)
sphere_cell_3 = openmc.Cell(region=-sphere_surf_3)
sphere_cell_3.name= 'sphere_cell_3'

sphere_cell_2.fill = mat_iron
sphere_cell_3.fill = mat_aluminum

my_materials = openmc.Materials([mat_iron, mat_aluminum])
all_cells = [sphere_cell_1, sphere_cell_2, sphere_cell_3]
universe = openmc.Universe(cells=all_cells)


mat_colors = {
    my_materials[0] : 'green',
    my_materials[1] : 'gray'
}
cell_colors = {
    all_cells[0] : 'green',
    all_cells[1] : 'gray',
    all_cells[2] : 'yellow'
}

universe.plot(
    # legend=True, 
    width=(44, 44),
    origin=(0., 0., 0.),
    outline=True,
    colors=cell_colors,
    color_by='cell'
)
plt.savefig("plot_cell.png", bbox_inches="tight")

universe.plot(
    # legend=True, 
    width=(44, 44),
    origin=(0., 0., 0.),
    outline=True,
    colors=mat_colors,
    color_by='material'
)
plt.savefig("plot_material.png", bbox_inches="tight")

[shimwell]

looks like the CI is failing for reasons unrelated to the PR, looks like there is a fix in already so I can rerun the CI after #2481 has been merged in

[shimwell]

canceled the CI for the previous run as I had one more commit to sneak into this PR. Latest commit just improves the pytest a little bit

[paulromano]

All looks good now; thanks @shimwell!

[paulromano]

@shimwell The merging of #2472 caused a conflict here that you'll need to resolve