Add option to toggle show_pml for Mesh

[ALEEF02]

The PML display is clunky in AppCSXCAD, reducing the visibility of the actual structures. This toggle in Mesh allows the user to hide it.

Use:

mesh = Mesh(
    sim=sim,
    metal_res=1 / 80,
    nonmetal_res=1 / 40,
    min_lines=3,
    expand_bounds=((0, 0), (0, 0), (20, 20)),
    show_pml=False
)