`model_add_hvac_system` in nze_hvac measure

[JanghyunJK]

for this line (and actually for four more occurrences),

https://github.com/NREL/openstudio-ee-gem/blob/5fa607ec2f7277cff2617d0e203d5c71fdf5c6d2/lib/measures/nze_hvac/measure.rb#L89

is using self correct (for the first arg)?
standard.model_add_hvac_system(self, ...

or should it be using model like this?
standard.model_add_hvac_system(model, ...