make simper work with single-member groups; make some of the loop counters robust

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2874 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
fix a typo in part of the fix to r2874

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2875 edb9625f-4e0d-4859-8d74-9fd3b1da38cb