1. Centre Inria d’Université Côte d’Azur, 06902 Sophia Antipolis, France.
2. Department of Mathematics, RPTU Kaiserslautern–Landau, 67663 Kaiserslautern, Germany. Former address: Centre Inria d’Université Côte d’Azur, 06902 Sophia Antipolis, France.
3. Department of Mathematics, UiT The Arctic University, 9037 Tromsø, Norway.