[multidomain] Only emit parallel residual warning if more than one core

dumux/multidomain/fvassembler.hh

@@ -254,7 +254,7 @@ public:
             }
             else if (!warningIssued_)
             {
-                if (gridView.comm().rank() == 0)
+                if (gridView.comm().size() > 1 && gridView.comm().rank() == 0)
                     std::cout << "\nWarning: norm calculation adds entries corresponding to\n"
                               << "overlapping entities multiple times. Please use the norm\n"
                               << "function provided by a linear solver instead." << std::endl;