box model: use correct names for convergence output

counter-intuitively, ostringstream::clear() does not reset the string
it writes to. that can be done using ostringstream::str("")

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7331 2fb0f335-1f38-0410-981e-8018bf24f1b0

dumux/boxmodels/common/boxmodel.hh

@@ -644,11 +644,11 @@ public:
 
         for (int i = 0; i < numEq; ++i) {
             std::ostringstream oss;
-            oss << "x_" << i;
+            oss.str(""); oss << "x_" << i;
             writer.attachVertexData(*x[i], oss.str());
-            oss.clear(); oss << "delta_" << i;
+            oss.str(""); oss << "delta_" << i;
             writer.attachVertexData(*delta[i], oss.str());
-            oss.clear(); oss << "defect_" << i;
+            oss.str(""); oss << "defect_" << i;
             writer.attachVertexData(*def[i], oss.str());
         }