Add F9_00_ORG_EXEMPT_TYPE to the 990 header table
Source: R/08_extract_csv_tables.R
add_exempt_type.RdJoins the status derived by exempt_type_sql() onto the header table and
places it after the F9_00_EXEMPT_STAT_* checkboxes it summarises. It lives
here rather than in KEYS so that only F9-P00-T00-HEADER changes, and so
that existing archives gain it on a table rebuild, with no re-parse.