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

[mlbernauer]

In total there appear to be 5732 distinct non-null NDCs in the hosp.prescriptions table, I was able to map each NDC to at least one of the class vocabs, however some vocabs have more coverage than others. Here's a breakdown of how the NDCs from the prescription table map across the different vocabularies available within RxClass

vocab	mapped_ndcs	total_ndcs	fraction_mapped
MEDRT	5152	5732	0.898813677599442
ATCPROD	5119	5732	0.893056524773203
VA	5061	5732	0.882937892533147
ATC	4902	5732	0.85519888346127
SNOMEDCT	4838	5732	0.844033496161898
FDASPL	4353	5732	0.759420795533845
DAILYMED	4331	5732	0.755582693649686
FMTSME	1164	5732	0.203070481507327
NA	579	5732	0.101011863224006
RXNORM	394	5732	0.0687369155617586

[mlbernauer]

As an example, one can use the ATC classification to query all NDCs for Antiinfectives for Systemic Use which has a top-level ATC classification code of J

select * from [mappings.drug_class] where relation_source like 'ATC%' and class_id like 'J%';

which should return 2109 results (785 distinct NDCs) across both the ATC and ATCPROD vocabularies.

One limitation of this mapping file is that it only contains mappings between drugs and their immediate parent class. In other words, if a drug maps to class A and class A maps to class B there will no be relation between the drug and class B. However, this limitation can be circumvented when using the ATC vocabs since their class_id codes are "semantic identifiers" i.e. they contain information about the class hierarchy which can be exploited as in the example above