Sudan, torn by civil war, human misery and upheaval, was not always that way. For centuries Africa's biggest country was a jewel in the continent's cultural crown - rich, powerful and a meeting point between the Mediterranean, southern Africa and the Middle East.In a bid to offset the modern image of violence and misery epitomised by the crisis in the western region of Darfur where thousands of people have been killed and more than a million uprooted, the British Museum is staging a major exhibition.