Medcode

- new features: support adding coding systems later, e.g.,

the initialization stage
tool = CodeMapping('NDC', 'RXCUI', 'Name', 'SMILES')
tool.load_mapping()

add the "RxNorm" coding systems later
tool.add_code("RxNorm")
tool.NDC_to_RxNorm['76413-153-06'] example

First Medication Coding Mapping Tool