Implement interface conditions based on bulk and surface conductivity

Currently, SSD can take into account interface conditions (e.g. between a semiconductor detector and the outside medium) based on the change in relative permittivity, that will result in an alteration of the electric field and an effective charge up of the surface (see https://en.wikipedia.org/wiki/Interface_conditions_for_electromagnetic_fields for reference)

Another set of interface conditions that SSD is currently not taking into account is based on surface conductivity (see section II in https://doi.org/10.1109/NSSMIC.2001.1008448), that would alter Gauss's law at the interface to Eq. (6) in said paper.

This might require some work, but it would be nice to implement this interface condition in SSD and let the user choose to regard this or not.