Future research: map to UMLS concept ids

[rkboyce]

For research : Currently we map all concepts to RxNorm and SNOMED-CT. In the ETL process, we currently map directly from source vocab to these. When these mappings don't exist, it might be effective to map to UMLS MetaThesaurus concept first, and then to the most specific concept in the target vocab that we can get to through the concept and its synonyms. This might be an opportunity to use concept_similarity too.