Add demo for solving axisymmetric Maxwell's equations - GSoC 2022

[mikics]

Currently, there are no demos showing how to solve axisymmetric electromagnetic problems. This pull request adds a demo for showing how to solve a simple problem of such kind: a metallic sphere hit by a plane wave (with axisymmetric PML condition). By exploiting the cylindrical expansion for a plane wave, we can split the full 3D problems into few 2D problems, with an overall significantly lower computational cost.