use of "abstract" classes

@boerrebj: Abstract classes (partial classes in Modelica terminology) are useful for collecting common parts of multiple detailed models. As an example,

• AbstractPipe basis class for both rigid water column pipe as well as elastic water pipe
• AbstractValve basis class for different types of valves
• AbstractTurbine basis class for different turbine types

I would probably tend to call those:

• BasePipe
• BaseValve
• BaseTurbine

One possible alternative could be: AbstractTwoPortElement ->

• AbstractPipe
• AbstractSingularLossElement (typical for trash racks, contractions/expansions etc.)
• AbstractControlledElement (typical for both gates and valves)
• AbstractTurbine