1 Department of Electrical, Electronic and Information Engineering "Guglielmo Marconi", University of Bologna, 47521 Cesena, Italy.
2 MOX - Dipartimento di Matematica, Politecnico di Milano, 20133 Milan, Italy.
3 Department of Electrical, Electronic and Information Engineering "Guglielmo Marconi", University of Bologna, 47521 Cesena, Italy; BioMediTech, Faculty of Medicine and Health Technology, Tampere University, FI-33520 Tampere, Finland.
4 MOX - Dipartimento di Matematica, Politecnico di Milano, 20133 Milan, Italy; Mathematics Institute, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.