Use of anisotropic materials in RayFlare

[phoebe-p]

Is your feature request related to a problem? Please describe.
It should be possible to use RayFlare with anisotropic materials (i.e. pass a dielectric/permittivity tensor rather than simply constants). In fact, S4 is capable of dealing with anisotropic materials, but it is not currently possible to pass such material data to it from RayFlare/Solcore.

Describe the solution you'd like
Adjust OptiStack and the relevant functions in rcwa.py to allow passing a dielectric tensor (wavelength-dependent).

Additional context
[] Issues occur in interpolation (interpolating a multi-dimensional rather than 1D array)