Experimental release: adding stem table to RiaH

This is an experimental release, introducing the notion of a stem table to RiaH. The stem table is a virtual intermediate table (that may or may not have a, equivalent in the real ETL) that is the origin for all CDM tables where assignment to that table is predicated on the domain of the concept ID that a record was mapped to.

For example, ICD-9 codes can map to concepts in either the condition or procedure domain, and ICD-9 diagnoses therefore need to be mapped to the condition_occurrence or procedure_occurrence table, depending on the domain of the mapped standard concept. Rather than specifying two mappings in RiaH, only one mapping to the stem table needs to be specified by the user. Default mappings from the stem table into the condition_occurrence and procedure_occurrence tables are already provided.