Outsource Mobility Models to Separate Classes

This is a first attempt to separate the charge carrier mobility model we used so far (Jacoboni et al) from the different propagation module codes and provide a central instance of it.

Furthermore, it adds new mobility models (Hamburg, Hamburg High Field) which can be selected via a parameter in the propagation modules.

There are a few things to do:

• Write a separate manual section about the mobility models and refer to it from the modules
• Rename physics folder, that's not very nice.
• Figure out how we can work around the zero mobility and e-field usage in ProjectionPropagation

At a later stage we might want to extent this to

• Include doping/dopant dependent mobility models (Arora, Masetti)
• Also provide separate classes for other models used (lifetime, impact ionization)