added validity test for species<- data.frame method

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegsoup@660 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
improved vertical argument

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegsoup@661 edb9625f-4e0d-4859-8d74-9fd3b1da38cb