NEW YORK (Reuters) - New Yorkers often become part of unexpected mob scenes -- huge crowds on subway platforms, at clothing sales, or at free concerts in places like Central Park. Now they are doing it on purpose, for fun. "Flash mobs," in which people show up at an assigned place at a certain time, perform some brief acts, and then leave, have descended on stores, a hotel and even a piece of a park in New York. In the latest occurrence, about 200 people converged on a Central Park ridge across from the Museum of Natural History on Thursday....