restructuring - use tmpcfg=keepfields and rollback_provenance for ft_volumedownsample. See http://bugzilla.fcdonders.nl/show_bug.cgi?id=2596


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@9654 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0