implement Mahalanobis, Manhattan and Gower metrics for bioenv

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2499 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
rankindex gained Mahalanobis and Manhattan metrics for gradients

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2500 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
add Mahalanobis to vegdist

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2501 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'r-forge-svn-local'