cluster.index with 'mat' argument for stacking rows of a matrix according to cluster-variable

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/lava/pkg/mets@896 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'master' of github.com:kkholst/mets