Handle setting system capacities and efficiencies when the design load and airflow is zero

[lymereJ]

There is a larger problem of how to handle setting system capacities and efficiencies when the design load and airflow is zero. It's a source of errors in other models too.

One option is to set a minimum size for each kind equipment. It could be a universal size minimum, say 1/2 ton, ~1.5 kW, which is about the smallest equipment size one can find for space heaters or residential a/c units.

A second option is to remove equipment entirely if there is no load. I prefer this option, but it may mean a lot of tricky clean up for model objects if the user selects complex HVAC systems with EMS controls for small zones with no load.

Originally posted by @mdahlhausen in #1476 (comment)