Add chartevents concept mapping

[a-chahin]

This pull request adds two mapping tables for chartevents. The first CSV file meas_chartevents_main contains itemid to omop_concept_id and omop_concept_code for either LOINC or SNOMED depending on the concept. The second CSV meas_chartevents_value file is for values, concepts that do not have an itemid. The values are also mapped to omop_concept_id and omop_concept_code for either LOINC or SNOMED. Mapping codes were reviewed and verified by @xborrat.