Add a conformance requirement to capture the best practice that terminology membership is preferred over value set evaluation.

Specifically, authors SHOULD avoid the use of expressions that require implementations to perform the expansion of a value set, preferring instead the use of the terminology membership operator. For example, rather than combining multiple value sets using a `union`, separate membership tests in each value set should be used.

Refer to the following github issue for more information:

https://github.com/cqframework/clinical_quality_language/issues/179