Observations for numeric concepts should also support exceptional values

(Original Request)

Labs typically need to use comments to describe null results for numeric observations due to machine or sample errors. Currently we cannot set a ConceptNumeric to null without generating an APIException, so we need a new concept type that is typically numeric but can handle being set to null. If it is null, it must have a comment to be valid. Obs of this concept type must be displayed by showing the comment along with the value, especially if it is null. Displaying the comment when the value is not null may not be necessary.

(Darius's Comment)

We shouldn't need a new type for this, rather we should provide a way for the standard numeric datatype to support exceptional values like "specimen contaminated".