Refactor {xml-id sql-id} context format
Accumulate contexts stack in insert-table
resolve-fn in *-ref functions for stack traversal

 - The default `resolve-fn` is `first`
Import drug references in treatment activities

 - Using new "further up the stack" reference resolution
Import: add units, treatment-dosings, designs
Fix import of study start/end dates
Allow measurements on overall population (magic arm)
post-insert hooks to generate RDF
Add hypertension import script
Cleanup