Revisit CQL Engine and Result APIs

[JPercival]

The CqlEngine APIs were created in an ad hoc fashion, as we were uncertain what an optimal API would look like. This has resulted in a few cases where the APIs are inconsistent, don't follow Java best practices, or suffer from code smells such as "telescoping constructor", or having many function overloads.

The engine still isn't API complete/final. Specifically, we don't have all the debugging APIs in place, so any current work on updating the APIs will be a step along the way.

Example of many overloads (potentially addressed by a params class):
https://github.com/cqframework/clinical_quality_language/blob/master/Src/java/engine/src/main/java/org/opencds/cqf/cql/engine/execution/CqlEngine.java#L110-L176

Example of brittle API, which is not extensible as new retrieve options are added:

clinical_quality_language/Src/java/engine/src/main/java/org/opencds/cqf/cql/engine/retrieve/RetrieveProvider.java

Line 6 in e3171ce

public interface RetrieveProvider {

Example of improper hiding:

clinical_quality_language/Src/java/engine/src/main/java/org/opencds/cqf/cql/engine/execution/EvaluationResult.java

Line 9 in e3171ce

public Map<String, ExpressionResult> expressionResults;

This story is a place holder for a design task to revise these to be more useful/consistent.