much faster stacking with new mets::cluster.index. Examples updated

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/lava@894 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
doc

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/lava@895 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
cluster.index with 'mat' argument for stacking rows of a matrix according to cluster-variable

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/lava@896 edb9625f-4e0d-4859-8d74-9fd3b1da38cb