Unifying optical properties of layers (refractive index, form factor, epsilon, ...)

[dschick]

The udkm1Dsim toolbox was initially developed to simulate laser-pump/X-ray probe experiments.
For that purpose, layers have the opt_ref_index property to calculate the (pump) laser absorption as well as atomic and magnetic form factors provided by the actual Atoms in the layer to calculate the X-ray scattering.

Since we also want to simulate light scattering in the visible and more generally in any spectral range (same is true for the laser excitation) it is necessary to unify the definition and also how the optical properties are provided.

I suppose that the most general form for the optical properties is an arbitrary permittivity tensor, which should also be the backend within the udkm1Dsim. The must be provided for one or more photon energies, which then allows for interpolation for any photon energy requested by the user.

In order to set the permittivity tensor, we have to provide a set of methods, e.g. to read from a file and also manually provide values. Of course this must be flexible enough to cope with different formats and inputs, like refractive indices and form factors as well as dichrosim and birefringence.