enhancement - trim the output in the presence of multiple input arguments and implemented cfg.selmode to be 'intersect' (old and default behavior) or 'union' (new, replaces missing data with nans

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@9199 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0