While many investors cursed the five-year downturn in European stock markets, activist hedge fund manager Cevian Capital had a reason to celebrate. Poorly managed European companies with declining profits were more receptive than ever to outside help from activist firms, like Cevian, that can intervene to help turn their fortunes around. The London-based manager has profited handsomely from the boost in business, enjoying a $4.69 billion jump in assets between 2013 and 2015. It is now the ninth-largest hedge fund in Europe and the continent's largest so-called constructivist activist hedge fund manager, meaning that it invests in companies whose management is receptive to advice and long-term operational changes rather than short-term portfolio trading…