More prep for building sizer 6

[k-rieck]

• use simple_water_storage for dhw instead of hot_water_storage_modular because dhw so far only worked with the generic_heat_source component which exists only for gas, oil and district heating
• use the each individual heating component now with dhw simple_water_storage to extend it more easily and to have dhw and space heating modulated similarly
• todo: heat pump dhw needs to be adapted still