Provide API for accessing 'internal' nodes for 'virtual' HVAC components in model

Enhancement Request

For 'virtual' components that can be placed on an AirLoopHVAC, node setpoint control is not available because the nodes are not created until forward translation. An API that exposes or enables referencing these nodes would allow for defining more detailed HVAC controls.

@kbenne describes this as 'tricky' here, but I believe it would be highly valuable in enabling complex HVAC controls. I've talked with @jmarrec about wanting this feature and he probably has ideas on how it could happen.

Detailed Description

Here is a (probably incomplete) list of where the internal node limitation crops up:

• AirLoopHVAC:UnitarySystem control type SetPoint requires set points at each coil outlet node, which is not possible in OpenStudio because the nodes don't exist until translation.

• ZoneHVAC:TerminalUnit:VariableRefrigerantFlow can be placed on an AirLoopHVAC (or AirLoopHVAC:OutdoorAirSystem or AirLoopHVAC:DedicatedOutdoorAirSystem), for which the unit control is based on setpoints placed on the coil outlet nodes. Also not possible currently.

• The cooling coils in CoilSystem:Cooling:DX:HeatExchangerAssisted and CoilSystem:Cooling:Water:HeatExchangerAssisted cannot have a humidity setpoint (or any, for that matter) added to the coil outlet node, because it doesn't exist.

Possible Implementation

If, for example, an object ZoneHVAC:TerminalUnit:VariableRefrigerantFlow has a method coolingCoil, could there also be methods coolingCoilInletNode/coolingCoilOutletNode that would point to (optional) Node objects (or some related class without all the Connection/Port stuff) that SetpointManagers can reference, and would be translated accordingly?

Start by tracing forward translation for virtual AirLoopHVAC components and how their internal nodes are created. Review the listed cases—UnitarySystem, VariableRefrigerantFlow terminal units, and heat-exchanger-assisted cooling coils—and determine how optional node references could be exposed to SetpointManagers. Done means the API supports these cases and translates the referenced nodes correctly.