changed plot args of sts once more s.t. there now is xaxis.tickFreq, xaxis.labelFreq and xaxis.labelFormat which all take strptime configuration strings as input. A new surveillance options stsTickFactors controls the tcl sizes. ToDo: Document and check functions for daily data

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/surveillance@826 edb9625f-4e0d-4859-8d74-9fd3b1da38cb