Fix DSOA related methods in ZoneHVACIdealAirLoadsAirSystem to conform to API

[jmarrec]

The ZoneHVACIdealAirLoadsAirSystem does not conform to other ModelObject API and is currently an odd duck.

Todo:

• Remove IDD Field for Design Specifcation Outdoor Air Object: this should be infered from the classic inheritance schema (zone -> spaces() -> spaceType)
• Remove ZoneHVACIdealLoadsAirSystem::setDesignSpecificationOutdoorAirObject
• Rename ZoneHVACIdealLoadsAirSystem::designSpecificationOutdoorAirObject to designSpecificationOutdoorAir to match other objects API (such as Space/SpaceType)
• Modify implementation of ZoneHVACIdealLoadsAirSystem::designSpecificationOutdoorAir to find the appropriate Optional DSOA based on thermalZone->spaces()->designSpecificationOutdoorAir, like it's currently done in the Forward Translator: ForwardTranslateZoneHVACIdealLoadsAirSystem.cpp#L178:L198

Found while answering on unmethours here: https://unmethours.com/question/30924/outside-air-ideal-loads/