Allow changing temperature setpoint for heat load calculation in PartialZone

[jelgerjansen]

IDEAS.Buildings.Components.Interfaces.PartialZone calculates the design load for the zone as the sum of transmission losses QTra_design, infiltration losses QInf_design, and a term related to reheating after night setback QRH_design. QTra_design and QInf_design assume an indoor temperatuur of 21°C which cannot be changed. Since the room setpoint temperature differs between different rooms (e.g. living room vs bedroom), we might consider to add a temperature parameter for calculating the design heat load. This value should also be propagated to the different constructions connected to the zone, since the tranmission losses of every building component is calculated in those components.