extended meaning of layer argument

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegsoup@612 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
minor changes

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegsoup@613 edb9625f-4e0d-4859-8d74-9fd3b1da38cb