Restructure ThermalZone class (especially calculation)

Created by: PRemmen

Currently the ThermalZone class is rather large holds lots of functions and attributes that are not needed for standard case (e.g. all calculation attributes for four-element model). The idea is to separate the calculation of the parameters from the ThermalZone by creating four new classes (one for each model) and saving functions and attributes related to the calculation method in that class. This class will then be attached to the ThermalZone with an attribute.

We could also extend the lists for outer, inner walls to alls possible type (e.g. list for outer_walls, ground_floors and rooftops).

The API of thermal zone, building, and project class should not change