made agopenSized() function to create Ragraph object with size determined by graphviz; grid.graph() now creates viewport of width and height determined by Ragraph's boundBox

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@10 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
revised calculation of arrow length

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@11 edb9625f-4e0d-4859-8d74-9fd3b1da38cb