1. Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland
2. Department of Applied Mathematics, The University of Western Ontario, Middlesex College, 1151 Richmond St. N., London (ON), Canada N6A 5B7
3. Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland, Department of Applied Physics, Aalto University School of Science and Engineering, P.O. Box 11100, FI-00076 Aalto, Finland, and MEMPHYS-Center for Biomembrane Physics, University of Southern Denmark, Odense, Denmark