1. Corresponding author. Department of Mathematics, University of Oslo, Oslo 0851 Norway and Department of Business and Management Science, NHH Norwegian School of Economics, Bergen 5045 Norway.
2. Department of Probability Theory, Statistics and Actuarial Mathematics, Taras Shevchenko National University of Kyiv, Kyiv 01601 Ukraine and Division of Mathematics and Physics, Mälardalen University, Västerås 722 20 Sweden.
3. Department of Mathematics, University of Oslo, Oslo 0851 Norway and Statkraft AS, Oslo 0216 Norway.