Improve ETL of coded facts. Add codesystem to terminology mapping.

Instead of calling per-row dimension lookup functions, the most expensive
lookups are now replaced with joining the dimensions in the range table
instead.  For the sake of loose coupling, the API functions xxx2set_nk are
still used, but a bit rewritten to be better performing, at the loss of
generality.

This commit adds the codesystem to the terminology mapping table.  Todo is
adding the codesystem version.
Add loading staged data to PostgreSQL databases.

A new script was added that takes a gpload yaml script, and merges or inserts
data to the dimensions and fact tables in a PostgreSQL database.

Since file_fdw is used, this commit creates a dependency to PostgreSQL >= 9.1.