Fixed 'unbalanced' bug in vcovG


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/plm@116 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Added news of vcovG bugfix


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/plm@117 edb9625f-4e0d-4859-8d74-9fd3b1da38cb