added documentation for graphWidth and graphHeight

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@19 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added arguments to agopenSized documentation

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@20 edb9625f-4e0d-4859-8d74-9fd3b1da38cb