1. Department of Environmental Science , Aarhus University , Roskilde , Denmark
2. Institute of Information and Communication Technologies , Bulgarian Academy of Sciences , Sofia , Bulgaria
3. Institute of Mathematics , Budapest University of Technology and Economics , Egry J. u. 1., 1111 , Budapest , Hungary
4. MTA-ELTE Numerical Analysis and Large Networks Research Group ; and Department of Applied Analysis and Computational Mathematics, Eötvös Loránd University , Budapest , Hungary