Add columns default_domain and domain to stem table.

[don-torok]

We ETL into the Stem table, and then create views representing the OMOP tables, Condition, Drug Exposure, ... We use COALESCE(domain, default_domain) to determine which table a Stem table row belongs to. Where the domain is determined by the domain_id of the concept id, and the default domain is where the ETL determines the Stem table row should go if concept id is zero.