Adding value_source_concept_concept_id field to the Measurement and Observation table

[Alexdavv]

This is the formal issue for a proposal described in the presentation here with the CDM changes coded here.

[clairblacketer]

suggestion from CDM WG: Document how the two source concept ids (observation_source_concept_id and value_source_concept_id) work together to map to standard observations and values. Then the CDM needs to document the straightforward mapping of value_source_concept_id to value_as_concept_id