EnergyConversionSystem as abstract class

Created by: JoachimBenner

In the actual version, we declared _EnergyDistributionSystem and _StorageSystem as abstract classes, because we want to ensure that only the specializations like ThermalDistributionSystem are instanziated in an EnergyADE document. Is there a reason why we treat EnergyConversionSystem in a different manner? There are many specific classes for energy conversion systems being derived from the base class EnergyConversionSystem, but it is still possible to use the base class.