Add RxNorm

[gaurav]

RxNorm is a definitive source for drug information, and so we need to include RXCUIs in cliques.

The general algorithm we should use is:

1. Include all RxNorm entries, e.g. https://mor.nlm.nih.gov/RxNav/search?searchBy=RXCUI&searchTerm=204442
2. Consider them identical to all Ingredient (IN) identifiers -- in this case, RXCUI 5640 "ibuprofen". But note that this does not work with MINs, which work the other way, and tell you which drugs include this one.
3. Add additional names from RxNorm -- trade names, formulations, etc -- as synonyms of this concept.
4. Look for additional information in UMLS (e.g. https://uts.nlm.nih.gov/uts/umls/vocabulary/RXNORM/5640) such as trade names, formulations, etc.

This will have the side-benefit of getting rid of a lot of e.g. "Ibuprofen 50MG" entries in NameRes, since they would be cliqued into their ingredients. We could also extract RxNorm-UMLS and RxNorm-MeSH mappings (maybe to other sources as well: ChEBI? UNII?) so that we clique them together with other drugs.

Would close TranslatorSRI/NodeNormalization#127