It’s as if millions of people in coffee shops across the United States were using their laptops to do mathematical calculations while sipping their lattes — more than a billion calculations a second for each person. That’s how Rick Stevens, an associate director at Argonne National Laboratory in Lemont, Illinois, explains the unprecedented computational power of the lab’s new supercomputer. Dubbed Aurora, the $500-million device will be delivered to the lab in 2021, its creators announced Monday.