tokens: AR is indebted to Programa Banco de Santander for a UAB-distinguished postdoctoral research contract. Financial support from MINECO (projects CTQ2015-62635-ERC and CTQ2014-60119-P) and DIUE (project 2014SGR482)are gratefully acknowledged. NB acknowledges the financial support from the Université Joseph Fourier de Grenoble and the Observatoire de Grenoble, CC acknowledges the financial support from the French Space Agency CNES.
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