Adding External Heat Sources

[nealkruis]

We need to adapt heat source configuration to accept external heat sources (e.g., air-to-water heat pumps or [eventually] boilers). Here's a rough draft (but needs to be fleshed out more):

ExternalHeatSource:
    heat_source:
        Data Type: (RSAIRTOWATERHEATPUMP, RSBOILER)
    pump: # Add in the future when pumps are separate representations
        Data Type: RSPUMP
    control_type:
        Data Type: <ExternalHeatSourceControlType> # Enumeration of FixedFlowRate (for multi-pass systems) or FixedOutletTemperature (for single-pass systems)
    fixed_flow_rate:
        Data Type: Numeric
        Units: m3/s
        Required: if control_type=FixedFlowRate
    fixed_outlet_temperature:
        Data Type: Numeric
        Units: K
        Required: if control_type=FixedOutletTemperature

Note: This data group does not represent a product directly (it is not a Representation Specification itself)--and does not have its own Metadata or Description data groups.