“ The Christophers ,” the story of a past-his-prime painter (Ian McKellen) and the mysterious assistant (Michaela Coel) he hires to destroy some priceless works of his half-finished art, defies easy categorization. It’s funny and sad, veering between a crime thriller and a character drama, as it examines the precarious nature of talent. Why, it asks, do some artists lose their creative spark? “We didn’t really think about genre,” says Steven Soderbergh , the film’s director. “Human behavior was