Minor edits
Convert procedural logic of data loading into a single CTE.

Use of Common Table Expression reduces the back-and-forth between
PL/PGSQL engine and the SQL execution engine.

It also allows for us to later convert this into a SQL-language
function.
Added data load for CUSTOMER table, and first bug-fix.

Aparently I had forgotten to add the C_YTD_PAYMENT column to the
CUSTOAMER table to begin with! This bug was caught While adding the new
code.