enhancement - allow the specification of the operation like (x1-x2)/(x1+x2), see http://bugzilla.fcdonders.nl/show_bug.cgi?id=2578


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@9524 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0