1. Research Center for Astronomy, Academy of Athens, Soranou Efessiou 4, 11527 Athens, Greece
2. Department of Electrical and Computer Engineering, School of Engineering, Democritus University of Thrace, University Campus, Kimmeria, 67100 Xanthi, Greece
3. Department of Mathematics “Tullio Levi-Civita”, University of Padova, Via Trieste 63, I-35121 Padova, Italy
4. Department of Physics, University of Athens, Panepistimiopolis, 11521 Athens, Greece