Acknowledgements
Work supported by the Spanish Ministry of Economy and Competitiveness, grant TIN2013-46638-C3-1-P, by Junta de Andalucía through projects P12-TIC-2541 and P11-TIC-7821 and by ERDF (FEDER) funds. A.D. Maldonado and R. F. Ropero are being supported by the Spanish Ministry of Education, Culture and Sport through an FPU research grant, FPU2013/00547 and AP2012-2117.