Create ObsReference tag

This would be a new tag that will be used to support integrating an encounter-based form with other obs collected during the same day under the same concept.

This will extend the standard Obs tag and support the following functionality:

• If there is an existing encounter associated with the context, will search for any other obs during that day that have a question concept equal to the concept specified on the tag; if there are multiple obs with the same concept, will take the most recent

• If a matching obs is found, and there is no matching obs on the actual encounter, will support substituting the obs in in view mode, or in view and enter/edit mode (this should be a configuration parameter)

• In edit mode, there should be a way to append a message to the widget displaying the reference value; you should be able to customize this message or suppress it; you should be able to include the

• Add a way to suppress the edit mode widget unless the the user clicks an "override" button

• Document this all