Update ppg_planner.c

to support SQL:
select
        sum(l_extendedprice) / 7.0 as avg_yearly
from
        lineitem,
        part,
        (SELECT l_partkey AS agg_partkey, 0.2 * avg(l_quantity) AS avg_quantity FROM lineitem GROUP BY l_partkey) part_agg
where
        p_partkey = l_partkey
        and agg_partkey = l_partkey
        and p_brand = 'Brand#41'
        and p_container = 'MED PACK'
        and l_quantity < avg_quantity
LIMIT 1;