add stressplot for prcomp and princomp

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2359 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
use row weights in stressplot of wcmdscale and cca

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2360 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
imaginary axes need similar adjustment as eigenvalues in capscale

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2361 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'r-forge-svn-local'