LONDON — The recent independence referendums in Iraqi Kurdistan and Catalonia, and the predictable heavy-handed responses from the central governments in Baghdad and Madrid, have raised many questions — a catechism without answers — on the meaning of nationhood in the 21st century. What is a nation? What is a nation-state? Is it the same as a country? Are a people, or a tribe, the same thing as a nation? In a globalized economy what does national sovereignty really mean?