\section*{Acknowledgments}
This work has been supported by MIUR project P.O.N.~RICERCA E COMPETITIVITA' 2007-2013 - Avviso n.~713/Ric.~del 29/10/2010, Titolo II - ``Sviluppo/Potenziamento di DAT e di LPP'' (project n.~PON02-00576-3333585). M.D., A.G. and F.V.P. acknowledge support by Istituto Nazionale di Fisica Nucleare (INFN) through the project QUANTUM. V.T. acknowledges the support, during the first part of this project, of the German Space Agency DLR with funds provided by the Federal Ministry of Economics and Technology (BMWi) under grant no. DLR 50 WM 1556.



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