Implement AirLoopHVAC::mixedAirNode()

This method was defined in the header but never implemented.
SDD: Support Fan:Pos on AirLoopHVAC

Now you can have a blow through or draw through fan position.  The
Fan:Pos element controls this, where previously the fan position was
dependent only on the fan element position in the SDD xml file.  Also
the controls (setpoint managers) are now arranged appropriately for blow
through control.