BOSTON (AP) — Running the Boston Marathon is tough enough without having to jostle your way from Hopkinton to Copley Square. So race organizers this year turned to an expert in crowd science to help them manage the field of more than 32,000 as it travels the 26.2 miles (42.195 kilometers) through eight Massachusetts cities and towns — some of it on narrow streets laid out during Colonial times. “There are certain things that we can’t change — that we don’t want to change — because they make the