It’s a land without light for months on end, with a reputation for gloom and of bleak landscapes that inspired 20th-century existentialism. But, art expert Andrew Graham-Dixon discovers, while its a fitting home for Munch’s The Scream, Scandinavia is also a land of eroticism, fairytales, futuristic thinking and an idealism that’s given us bicycle-riding monarchs, Lego and modernist social design.