Update DX coil models to represent heating mode and reversible systems

[AntoineGautier]

In Buildings.Fluid.HeatExchangers.DXCoils.UsersGuide, the following is mentioned.

Each stage has its own performance curve, which may represent the coil performance at different compressor speed, or the coil performance as it switches between cooling only, cooling with hot gas reheat, or heating only.

This sentence should be changed to "between cooling only or cooling with hot gas reheat". Indeed reversible / heating mode operation is not straightforward to represent with the existing models.

In #1723 (DOE2Reversible.mo) we support heating and cooling, but we switch the evaporator temperature to be medium 1 or 2 depending on the mode of operation, and use a built-in controller to set the set point for the outlet temperature if in heating mode. Something similar for the current DX coil model could be implemented to support generic split systems.

It might be better to use a completely separate model for the heating mode, and then use the existing DX coil for the cooling mode only.

[AntoineGautier]

@dhblum FYI this is the issue I opened after discussing the subject with Michael only a couple days ago.