bugfix - SeriesStep should not be NaN in case there is a single time point or frequency, see http://bugzilla.fcdonders.nl/show_bug.cgi?id=2735


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@9903 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0