Add flexibility for system-level design loads

[shorowit]

This issue arose as part of #1705.

HVAC systems can be connected to an HPXML Zone, which can have 1 or more Space children, and system design loads are reported at the zone level. That works well for the most common situation where heating and cooling equipment serve the same set of spaces. But it doesn't provide flexibility for other situations. For example, what if you have a two story building where a central furnace serves both stories but room ACs only serve the top story? You cannot construct a single set of spaces (zone) that is served by both.

Some possibilities:

• Allow an HVAC system to serve multiple conditioned Zones
• Replace HVAC/AttachedToZone with multiple HVAC/AttachedToSpace references
• Replace HVAC/AttachedToZone with Space/AttachedToHVAC references
• ???

Note that a workaround is to construct two different HPXML files, one for sizing the heating equipment and one for sizing the cooling equipment, but that's not ideal.

With any approach, we'll want to make sure it's not going to conflict with the EnergyPlus modeling should we ever decide to support multiple conditioned thermal zones and/or spaces.

[shorowit]

I was reminded that HPXML added the concept of Zones/Spaces to reflect how EGUSA models things. Indeed, that software appears to have the limitation noted here. So how do their users deal with this situation?