Department of Electrical Engineering and Computers, University of Coimbra, Polo II, 3030-290 Coimbra, Portugal and INESC Coimbra, Rua Antero de Quental, 3000-033 Coimbra, Portugal;Faculty of Economics, University of Coimbra, Av. Dias da Silva 165, 3004-512 Coimbra, Portugal and INESC Coimbra, Rua Antero de Quental, 3000-033 Coimbra, Portugal