zap almost-zero components in the print of cca/rda/capscale

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2627 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
plotting vectors failed if one variable was invariable and no vector could be calculated for it

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2628 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
envfit tries to do something sensible with insensible input (constant variable, one-level factor)

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2629 edb9625f-4e0d-4859-8d74-9fd3b1da38cb