The story comes into focus with the arrival of yet another legend, Frankenstein’s monster — or Frank, as he likes to be called. By MANOHLA DARGIS Mary Shelley was just 20 when her wild, mind-expanding first novel, “Frankenstein; or, the Modern Prometheus” (1818), was published. By the time she turned 25, she had suffered a series of unspeakable losses, including the death of three young children and her husband, poet Percy Bysshe Shelley. For its part, her famous creation kept spawning new…