Add concept map Postgres load scripts for MIMIC-IV and NW.

[danamouk]

This pull request adds PostgreSQL scripts for loading concept map data into both MIMIC-IV and NW databases.

For each database (mimic-iv/concepts_postgres/concept_map/ and nw/concepts_postgres/concept_map/), it adds 5 SQL scripts and a README:

• create.sql — Creates the mimiciv_concept_map / nw_concept_map schema with 8 tables:
  labevents_to_loinc, labevents_to_omop,
  prescriptions_to_rxnorm, prescriptions_to_omop,
  chartevents_to_loinc, chartevents_to_omop,
  procedureevents_to_snomed, procedureevents_to_omop

• load.sql — Loads mapping CSVs from concepts/concept_map/hosp/ and icu/ into the tables

• constraint.sql — Adds primary keys on subject_id to enforce uniqueness and ensure data integrity.
  Prescriptions tables are excluded because different drug names share the same NDC code, so subject_id is not unique.

• index.sql — Creates indexes on subject_id and object_id to speed up lookups and joins against the mapping tables

• validate.sql — Checks row counts against expected values

• README.md — Quickstart and step-by-step setup guide

These concept maps provide mappings from local hospital codes to standard terminologies (LOINC, OMOP, RxNorm, SNOMED) using the SSSOM format.