enhancement - ensure that ft_math works on {pos}_ori_time source structures, suych as those from sourcegrandaverage, see also bug 2185


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@9351 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0