Add a way to consider Kramer-Kronig relations in design

[gomezzz]

Feature

Desired Behavior / Functionality

The Kramer-Kronig relations describe some constraints on permittivity / refractive index that we may be able to model.

What Needs to Be Done

• Find mathematical description of relation for permittivity that is differentiable
• Incorporate into https://github.com/esa/NIDN/blob/main/nidn/training/model/model_to_eps_grid.py

How Can It Be Tested

• Look at permittivity and see if realistic
• Check if training works with it