Fix function whose returning value is unclear

[bpark1327]

Pull Request Description

Fix function whose returning value is unclear

Checklist

PR Author: Check these when they're done. Not all may apply. strikethrough and check any that do not apply.

PR Reviewer: Verify each has been completed.

• [x] ~OS-HPXML git subtree has been pulled~
• [x] 301/ES rulesets and unit tests have been updated
• [x] ~301validator.xml has been updated (reference EPvalidator.xml)~
• [x] Workflow tests have been updated
• [x] ~Documentation has been updated~
• [x] Changelog has been updated
• [x] openstudio tasks.rb update_measures has been run
• [x] No unexpected regression test changes on CI

[bpark1327]

@shorowit In addition to changing the function for foundation walls u-factor and r-value, I've changed the function for getting default boiler efficiency. This function should return thermal efficiency (Et) for commercial boilers and AFUE for residential boilers. One thing I'd like to remind you is that we have been using Et and AFUE interchangeably (i.e., we have been modeling Et as AFUE). Please let me know if you have any concerns on this point. https://github.com/NREL/OpenStudio-ERI/blob/ab7a3eb439b9b7dcf5d7e2add4e58448827217d8/rulesets/resources/ES_ZERHruleset.rb#L1794-L1871

[bpark1327]

~@shorowit Check HPXMLs in run-unit-tests failed. Please guide me on how to solve it.~