Polygon composite surface

[eepeterson]

This PR is the addition of a CompositeSurface class for a generalized 2D polygon defined by a series of points, assumed to be connected by linear segments and closed. Essentially this class acts like any other surface and obeys the CompositeSurface protocol. It decomposes any polygon into a set of convex shapes, the union of which is the negative halfspace of the polygon and the complement of the union is the positive half space. You can also easily offset the polygon to form nested regions using the openmc.model.subdivide function. The following couple plots show the output of code similar to the following

polygon = openmc.model.Polygon(rz_points, basis='xz')
p1 = polygon.offset(2)
p2 = polygon.offset(4)
p3 = polygon.offset(6)
p4 = polygon.offset(7)
p5 = polygon.offset(10)
p5.boundary_type = 'vacuum'
regions = openmc.model.subdivide([polygon, p1, p2, p3, p4, p5])
cells = [openmc.Cell(region=r) for r in regions]
geom = openmc.Geometry(cells)
geom.export_to_xml(remove_surfs=True)

Nested vacuum vessel defined by a series of RZ points

Zoomed in version of the above

Decomposition of polygon into convex subsets

There's still a little clean up that needs to happen and tests to write before this is really ready.

[paulromano]

Really cool new feature @eepeterson! Just a few small comments:

[paulromano]

@eepeterson There's just a few more comments to address and then this should be good to merge. Thanks for the updates!

[paulromano]

Thanks for the updates!