makeFastHvacGreatAgain

In addition to #309 (closed): We would like to implement some features to the FastHVAC to hopefully make it a little bit more consistent and stable.

One of the goals is to

Tasks:

• mflow shall be defined as flow variable again
• new model for a generic splitter with variable split ratio between the ports
• implementation of a leakage massflow depending on a nominal flow for all components
• new demand model with conservation equation
• restructure the package order (see also #564 (closed))