Adding parameter to select n-dopant

The electron mobility parameters differ for the masetti model (and thus also the masetti_canali if the n-dopant is different (either arsenic or phosphorous). I have observed differences in past simulations that may be related to this, and am currently running proper tests. This MR makes it possible to select either arsenic or phosphorous, when using either of those two models, by supplying the n_dopant keyword to the propagation module.