Renaming of PhysicalSystem Class

[atra94]

In my opinion, the name PhysicalSystem could be improved for more clarity on its purpose.

The PhysicalSystem is the part to combine all models of the subsystems (Motors, Loads, Converters etc.) into one and to execute the simulation.

Therefore I would suggest to name it "SystemModel" or "CompoundSystem".

I would also appreachiate more and better suggestions. 😃

[wallscheid]

Good initiative, but requirse bird's eye view on the entire could package. Renaming on global basis in the future.