Defrost modeling

[jmaguire1]

OS-HPXML has made some pretty extensive changes to the defrost model, to more realistically model reverse cycle defrost. We should see if we can update our current code to reflect the latest.

Our current defrost model:

OCHRE/ochre/Equipment/HVAC.py

Line 1016 in 4fc6a11

# Based on EnergyPlus Engineering Reference, Defrost Operation, for on demand, reverse cycle defrost

OS-HPXML latest method:
https://github.com/NREL/OpenStudio-HPXML/blob/d9e1aff05595d7c79f41b9b39865c043c65a0678/HPXMLtoOpenStudio/resources/hvac.rb#L5065

There's currently RESNET discussions about defrost modeling as well. What OS-HPXML has will lead us to more realistic cycling and timeseries results, so that's what we ought to implement regardless of what RESNET decides.