1. Department of Mathematics, University of Oviedo, Oviedo, Spain
2. Department of Information Technology, University of Oviedo, Oviedo, Spain
3. Department of Physics, University of Oviedo, Oviedo, Spain and Wooptix S.L., San Cristóbal de La Laguna, Spain
4. Instituto de Astrofísica de Canarias, San Cristóbal de La Laguna, Spain