take care of the additive constant in cmdscale (called by capscale)

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2354 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
stressplot.default works only with isoMDS results and says it loud

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2355 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'r-forge-svn-local'