(Photo: Julian Roeder) What exactly is an opera? Karlheinz Stockhausen spent much of the last quarter of the 20th century providing increasingly improbable answers to that question. “Mittwoch aus Licht,” composed between 1995 and 1997, contains a world parliament debating esoteric love, an audition for an orchestra whose musicians are scattered through space, a string quartet performing in four helicopters {…}