Clarify Notation of Impurity vs Charge Density

[lmh91]

Clarify notation of the terms of impurity vs charge density and the naming of the corresponding functions and variables.

[fhagemann]

This is a good point.
Right now, simulation.ρ is the effective charge density divided by ε0.
We might want to change the names of the fields and the corresponding functions (e.g. get_charge_density()) to avoid confusion.

[fhagemann]

@lmh91 and I have decided to rename ρ to q_eff_imp as it describes the effective charge q/ε0 resulting from the ionized impurities in the depleted region.