Impedance boundary condition for wave port mode solve

[sebastiangrimberg]

Current the eigenmode solve for wave ports supports only PEC (homogenous Dirichlet) or PMC (natural) boundary conditions. It inherits these from the parent 3D model, but this can lead to confusion when a 3D model surface has an impedance BC (conductivity, surface impedance, absorbing) and that does not get correctly handled by the wave port automatically. The current expectation is that the user can manually chose whether an impedance boundary maps to a PEC or PMC BC in the wave port using the "WavePortPEC" object, but this might be overly complex and correctly supporting impedance BCs in the 2D mode solve should not be too difficult.

This might help issues like #264 tries to solve in the future.