Make waveports able to discriminate different TEM modes.

[CosimoMV]

All TEM modes in a transmission line have the same propagation constant and so Waveports cannot distinguish between them.
For example, in a transmission line like a coupled stripline exist two different modes: odd (the currents in the strips are equal in amplitude but in opposite directions) and even modes (the currents in the strips are equal in amplitude and in the same direction). At the moment is impossible to use waveports in a similar case, because is impossible to excite odd- and even modes, but only some combination of them and probably some surface mode.

Different modes are related with different symmetries (see last posts of #251, but in that case the coupled microstrip has different propagation constants for odd- and even modes, because doesn't support true TEM modes.)

[hughcars]

Hi @CosimoMV,

Thank you for raising the issue, wave ports can be pretty tricky to configure right now, so we're always looking for improvements. If there are multiple modes with identical propagation constant, then the ordering of the modes in the waveport solve will indeed likely not be stable meaning choosing a fixed index in the configuration will give variable results. My understanding is this is because the solve is highly unlikely to uncover exactly the same ordering each time, as the ordering will be defined by numerical precision (x - ~eps vs x + ~eps).

I haven't been able to think of a definitively more satisfactory solution than the suggested symmetry tricks so far, but if you have some suggestions we'd be open to working with you on them. One possible idea might be in a tie break to sort by the power, as if I understand correctly, an even mode would have non-zero integrated Poynting vector, but the odd mode would might have a different value. This "lexicographic" sorting would however need a notion of equality between modes, which opens the door on tolerances for these choices, (how close do two propagation constants need to be before we deem them identical and re-order their indexes based on the power?)

Hugh