further fixes in the growth-curve vignette


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/opm@535 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
further additions to growth-curve vignette


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/opm@536 edb9625f-4e0d-4859-8d74-9fd3b1da38cb