Allow Connector:Splitter and Connector:Mixer to be used to create bypass branch on AirLoopHVAC supply side

[eringold]

Enhancement Request

The EnergyPlus example file '5ZoneSupRetPlenRAB.idf' demonstrates how Connector:Splitter, Connector:Mixer, and Duct components can be used to create a supply-side bypass branch on an AirLoopHVAC. The HVAC-Diagram output of this file shows the following (labels not shown because of broken HVAC-Diagram in 9.5):

Connector:Splitter, Connector:Mixer, and Duct are all implemented in OpenStudio, however it appears that Connector:Splitter on an air loop forces a dual-duct configuration of that loop, and it doesn't seem that Connector:Mixer can be added anywhere.

This file uses SetpointManager:ReturnAirBypassFlow to set flow on the bypass loop. That object should be added to enable bypass operation.

Implementation should support creating/adding components to a branch before the Splitter and after the Mixer, and on any of the parallel branches between the two. SetpointManager:ReturnAirBypassFlow references the AirLoop (not a specific node).