The authors thanks the Associate Editor and two anonymous referees for their comments
that greatly improved the quality of the final paper. The first and third authors thank
the “Ministero dell’Istruzione, dell’Universit`a e della Ricerca” (MIUR) and the “Consiglio
Nazionale delle Ricerche” (CNR), Italy, for the support given to this project. The second
author thanks the “Ministerio de Educaci´on y Ciencia” (TIC2003-05982-C05-02), Spain.
The computational experiments were executed at the Laboratory of Operations Research (LabOR) of the University of Bologna.