Calculate scattering matrix

[garrekstemo]

Right now the transfer matrix method is used. While the current research in transfer matrices tries to mitigate its shortfalls (singularities and numerical instabilities), scattering matrices avoid these from the start. It would probably be more efficient and definitely more broadly useful to the photonics community to use scattering matrices instead of transfer matrices. In any case, once the transfer matrix is calculated, the scattering matrix elements are still calculated in the end to get transmission and reflection. (see this discussion)

Proposal: keep the current transfer matrix code, but write a scattering matrix implementation and have both available.