moved gradient function to gmrf because it doesnt work

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/diseasemapping@902 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
git-svn-id: svn://svn.r-forge.r-project.org/svnroot/diseasemapping@903 edb9625f-4e0d-4859-8d74-9fd3b1da38cb