Justin Cartwright is the author of more than a dozen novels, a sundry collection whose settings include the writer’s native South Africa, his adopted England, the Middle East, Hitler’s Germany, and Richard Nixon’s San Clemente. Now, here is “Lion Heart,” a novel that follows the fortunes of two more Richards: the present-day, 32-year-old Richard Cathar, whose friends call him Richie, and the 12th-century Richard I, whose admirers call him Lionheart.