It was irresistible outdoors in New York's Central Park last summer. Now this revival of the groundbreaking '60s musical is confined to a stuffy Broadway theater — and it's still irresistible. Pay no attention to the frenetic actors jumping in and out of the audience. What counts are the jubilant, impassioned, rapturously melodic doings onstage. A period piece? Possibly, but a brilliant one.