Fix non-isotropic half space

[domna]

Fix the eigenvalue solving and sorting for the non-isotropic half space case.

[MarJMue]

Definitely some work to do:

Tried to recreate the example from Passler and Paarmann. I think the best way forward is to implement the generalized transfer matrix formalism proposed in the paper, to see where the problem is.

[MarJMue]

I found a mistake in the eigenvalue sorting and the example works now.
It still does not include a method to resolve any occurring singularities. But i think this issue can be closed.