Add mapping file to map `hosp.prescriptions.ndc` to various drug classes

[mlbernauer]

This PR adds a mapping file (drug_mappings.csv) which allows drugs in prescriptions table to be organized into different classes (e.g. therapeutic, pharmacologic, mechanism of action, pharmacokinetic, among others).

col_name	description
ndc	NDC (foreign key into prescriptions table)
rxcui	RxNorm CUI corresponding to NDC
relation	Relationship between rxcui and maps_to_rxcui
maps_to_name	Name of the ingredient the NDC maps to
maps_to_tty	RxNorm term type indicating the type of agent the NDC maps to
relation_source	Source vocabulary for the mapping
maps_to_rxcui	RxNorm CUI of the agent that the NDC maps to
class_type	Describes the type of class
class_name	Class name
class_id	Class ID from the source vocabulary