1. School of Informatics, University of Northumbria, Newcastle-upon-Tyne NE1 8ST, UK
2. Permanent address: Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria
3. Department of Physics, Metallurgy and Material Science, Technical University Clausthal, 38678 Clausthal-Zellerfeld, Germany
4. Centre for Mathematical Science, City University, Northampton Square, London EC1V 0HB, UK