COPENHAGEN, Oct 20 (Reuters) – Swedish state-owned Vattenfall said on Friday it had started construction on Denmark’s largest offshore wind farm in the North Sea, able to power 425,000 households. The 406 MW Horns Rev 3. Both are for the Nouvelle Route du Littoral coastal highway. One of the two contracts, led by Vinci, involves building France‚Äôs longest offshore viaduct and the other, which is led by Colas, is for…