Add IsExplicit function to check for non-default.

[josephtfrank]

The IsSet function returns true even if the value that's available comes
from the default. Rather than changing the behavior of IsSet, adding a
new IsExplicit function allows checking for non-default values without
breaking existing behavior. This is intended to address issue #276 among
other things.

[claassistantio]

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1 out of 2 committers have signed the CLA.

✅ josephtfrank
❌ pallabi-spectralogic
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