Associates the structure definition with a collection of Data Elements

This extension associates a structure definition with a data dictionary - a collection of data elements that any instance of data that conforms to the structure definition must also conform to. The value of the extension is a uri the defines a query that identifies the collection of relevant data elements This extension can be used on any structure definition that is a constraint on the resource types Observation and Questionnaire. THe profile specifies which elements are mandatory etc., but any elements in a resource that conforms to the structure definition must also conform to one of the defined data elements. For Observation: * Observation.code must be associated with a matching Data Element by DataElement.code. * The structure definition should bind Observation.code to a value set that has the same codes as the collection of data elements (though it can be narrower) * the unit and the quantity value should conform to the rules laid down in the matching Data Element For Questionnaire: * todo.