Acknowledgements

L.L. Ferrás and A.M. Afonso acknowledge the Project PTDC/EMS-ENE/3362/2014-POCI-010145-FEDER-016665—funded by FEDER funds through COMPETE2020—Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT— Fundação para a Ciência e a Tecnologia; I.P. L.L. Ferrás would also like to thank the funding by FCT through the scholarship SFRH/BPD/100353/2014. M.L. Morgado would like to thank the funding by FCT through Project UID/MULTI/04621/2013 and M. Rebelo would also like to thank the funding by FCT through Project UID/MAT/00297/2013 (Centro de Matemática e Aplicações).
