Allow HeatingSystem + HeatPump (cooling only) on a distribution system

[shorowit]

A user's HPXML has a heat pump (that is only used for cooling, FractionHeatLoadServed=0) plus a heating system attached to a distribution system, and gets the error: "Multiple cooling systems found attached to the distribution system".

We check for at most 1 heating and 1 cooling system attached to a distribution system (see the note here), and indeed it's not nuanced enough to realize that the HP only provides cooling. I'm not immediately sure if we did this because it was hard to allow, or just because we didn't anticipate this situation.

Two workarounds are possible for the user:

1. Model the heat pump as a central air conditioner
2. Use a second distribution system for the heat pump.

But it would be nice to allow this if possible. One less error for a user to have to deal with.

[shorowit]

I think the effort would be low to allow this?