Support Retrieve:codeComparator

[cmoesel]

The Retrieve operator provides a codeComparator to indicate how the code should be matched. According to chapter 4, section 11.1.14:

The codeComparator attribute specifies how elements of the code property should be matched to the terminology. One of 'in', '=', or '~'. Note that 'in' will resolve to the appropriate terminology matching operator, resulting in equivalence semantics for value set and code system membership testing.

It's possible that cql-execution generally does the right thing, but it would be better to explicitly read this property (when present) and use the semantic that it suggests.