eigenvector(): prevent getting stuck in iteration

This aborts the iteration if no progress is being made. This fixes issue #276.

src/graph/centrality/graph_eigenvector.hh

@@ -95,7 +95,7 @@ struct get_eigenvector
             ++iter;
             if (max_iter > 0 && iter == max_iter)
                 break;
-            if (max_iter == 0 && delta > prev_delta && iter > 100)
+            if (max_iter == 0 && delta >= prev_delta && iter > 100)
                 break;
         }