The work upon which this paper is based on was co-financed by COGERSA, S.A.U. (Projects FUO-EM-138-15, FUO-139-16), IDEPA (Project IDE/2015/000245), Spanish MINECO (Project CTM2015-63864-R) and FEDER funds from European Union. Technical assistance from the Scientific-Technical Services of the University of Oviedo is gratefully acknowledged
