Draft: Add Electric Field Option for Gain Layers

This is the first port of @fdewit's work on gain reduction mechanisms to the main repository.

All code is from @fdewit, my changes only concern

• Formatting
• Some readability & rebase to latest master
• Documentation, taken from @fdewit's thesis

For reference, the thesis can be found here: https://www.ru.nl/publish/pages/913454/2023_femke_de_wit_thesis.pdf

To do:

• Add test cases based on examples from @fdewit
• Ensure that this also works e.g. with an iLGAD sensor or with an n bulk sensor
• Finish documentation