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This work was supported by the grants PTDC/SAU-OSM/102907/2008 and PEst-C/SAU/LA0001/2013-2014, funded by FCT (Portuguese Foundation for Science and Technology) and FEDER/COMPETE (European Fund of Regional Development/Operational Program Factors of Competitiveness). Diana Serra is a recipient of the grant SFRH/BD/7541/2010 from FCT