added num_deriv() function

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@939 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
numerical derivatives testing

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@940 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
numerical derivatives testing

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@941 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
numerical derivatives testing

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@942 edb9625f-4e0d-4859-8d74-9fd3b1da38cb