In Ezekiel Emanuel’s “Eat Your Ice Cream,” the health policy expert and oncologist talks about his “six simple rules” for living longer and better. The wiry and intense 68-year-old is no scolding nutritionist, readily admitting his love for sweet treats — he ate banana splits as a kid and now regularly bakes apple crisps, banana cake and his mother’s cheesecake — but he indulges only a couple of times a week and in small portions.