betadisper(..., type = 'centroid') failed when there was only one group

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2484 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
redo fix applied in r2484 in a different way, print 'mediod' when using type = "median", reduce number of eigenvalues printed, add eigenvals method for betadisper

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2485 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
janitorial after r2485: betadisper medoid and print rewrite

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2486 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
update tests/

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2487 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'r-forge-svn-local'