Why is Observation.value constrained to just the CodeableConcept data type? The description in Section 3.2 specifically says "In the case of an assessment with a summary score, for example, this field may appropriately be used to contain the score value." but doesn't give guidance on converting a score to a codeable concept. It seems like for a use case like this, a data type of string, integer or Quantity might be more appropriate. Why constrain the data types at all?