EPWDesignCondition getters should return boost::optional

[joseph-robertson]

Issue overview

Missing design condition (double?) fields in the header of the EPW file return a zero, and they shouldn't.

See #3086 for the original implementation.

Current Behavior

Expected Behavior

Steps to Reproduce

Possible Solution

Details

Environment

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Context

[joseph-robertson]

Note that 2021 ASHRAE Handbook design conditions contain a new WSF element just before the cooling design conditions. Does this mean that the current version of the SDK's epw_file.designConditions doesn't support 2021 design condition headers?

See NREL/resstock#1137 for some context.

[shorowit]

That's not part of the EPW spec, right? So it shouldn't be included in any EPW files.