add theta-sketch intesection agg function

[phstudy]

add theta-sketch intesection agg function

example:

postgres=# create table theta_sketch_test(sketch theta_sketch);
postgres=# insert into theta_sketch_test select theta_sketch_build(1);
postgres=# insert into theta_sketch_test select theta_sketch_build(1);
postgres=# select theta_sketch_get_estimate(theta_sketch_intersection(sketch)) from theta_sketch_test;
 theta_sketch_get_estimate 
---------------------------
                         1
(1 row)

[AlexanderSaydakov]

Thank you!