Fixes for list Unions

[JPercival]

• Correct erroneous tests for list Union behavior
  • According to the spec for List unions null union {1, 2, 3} should return {1, 2, 3} and not null
  • https://cql.hl7.org/09-b-cqlreference.html#union-1
• Engine fixes and additional tests for the above
  • Resolves Union of lists with a null is incorrect #887
  • Partially resolves union/distinct operators do not match spec for lists when null elements are involved. #1132
• Added a link to the CQL spec in the Readme
• Fixed missing U of U copyright for the CQL engine

[brynrhodes]

Builds locally and tests pass, overall looks good, but I think there is a copy/paste error in the UnionEvaluator?