allowing varible offsets for polygon.offset

[shimwell]

Description

This PR allows the polygon offset distance to be an iterable of values for a non constant offset. I hear this is useful for stellerators :-)

Checklist

• I have performed a self-review of my own code
• 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)

example script

import openmc
import numpy as np
import matplotlib.pyplot as plt

star = np.array(
    [
        [1.0, 2.0],
        [0.70610737, 1.4045085],
        [0.04894348, 1.30901699],
        [0.52447174, 0.8454915],
        [0.41221475, 0.19098301],
        [1.0, 0.5],
        [1.58778525, 0.19098301],
        [1.47552826, 0.8454915],
        [1.95105652, 1.30901699],
        [1.29389263, 1.4045085],
        [1.0, 2.0],
    ]
)

surf_poly = openmc.model.Polygon(star, basis="rz")
surf_poly2 = surf_poly.offset(
    [
        0.5,
        0.1,
        0.1,
        0.1,
        0.1,
        1.0,
        0.1,
        0.1,
        0.1,
        0.1,
    ]
)

region_poly = -surf_poly
region_poly2 = -surf_poly2 & +surf_poly

cell_poly = openmc.Cell(region=region_poly)
cell_poly2 = openmc.Cell(region=region_poly2)

my_geometry = openmc.Geometry([cell_poly, cell_poly2])
my_geometry.plot(basis="yz", width=(8, 8), color_by="cell")

plt.show()

[paulromano]

Thanks @shimwell!