added coverscale pa and is.occurence method

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegsoup@647 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added compress method

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegsoup@648 edb9625f-4e0d-4859-8d74-9fd3b1da38cb