Close-to-degenerate guided modes

[momchilmm]

Currently the guided-mode computation assumes non-degenerate modes. The "resolution" in looking for those is given by gmode_step. The computation becomes really heavy when pushing gmode_step down to values below e.g. 1e-5. In some structures, like two identical (effective) slabs separated by an air gap, the modes can be almost degenerate, and very hard to resolve. There's no obvious solution to that but putting this here as something to think about in the future.