Improve inheritance structure

[fwitte]

Resolve #40

The idea is, to have a look at the different levels of abstraction in the __init__.py (potentially also other methods) structure and add the missing levels as pseudo-classes, which do not include any intelligence in terms of the simulation model, but mostly for data pre- and postprocessing.

I only commited changes to two cascade heat pumps to show, what the idea could look like.