Update heatpump testbench to AixLib 0.9.1

[jnettels]

The Modelica project heatpump testbench was initially based on AixLib 0.7.3. This shows how a tight integration of Python and Modelica is possible, e.g. preparing a simulation by swapping heatpump base data and simulation via Python.

With AixLib 0.9.1, there are currently two issues preventing a seamless update:

• AixLib 0.9.1 is not fully compatible with OpenModelica. AixLib.DataBase.ThermalMachines.Chiller.ChillerBaseDataDefinition causes an error in OpenModelica, while it works in Dymola. There is an easy workaround, but currently the authors seem unwilling to include it, since it makes the code somewhat less elegant. See issue "FastHVAC heat pump does not run in OpenModelica" 864 in https://github.com/RWTH-EBC/AixLib (I do not want to link directly, because that reference would be shown there). I tried to be as polite as I can possibly be, but I do not agree with their decision. Fortunately, we do not need the chiller function, so we can work around the issue.
• Replacing the heat pump data programmatically via Python is an essential part of the testbench. Due to changes to the heatpump model in 0.9.1, this data selection is now contained in a redeclare statement. This syntax of the Modelica standard is not fully supported by OpenModelica (Dymola handles it just fine), so there is no command to replace this information. See this for more details: https://stackoverflow.com/questions/62168453. An update with that feature is being worked on, but we can only wait patiently for it to happen.