use method applyQuadratureRule in mmdg

dune/mmdg/dg.hh

@@ -80,7 +80,7 @@ public:
   }
 
   //compute L2 error using a quadrature rule
-  Scalar computeL2error(const int quadratureOrder = 5) //const
+  Scalar computeL2error(const int quadratureOrder = 5)
   {
     if (!problem_.hasExactSolution())
     {
@@ -97,7 +97,7 @@ public:
       const QRuleElem& qrule = QRulesElem::rule(simplex, quadratureOrder);
 
       //determine L2-error on element elem using a quadrature rule
-      const auto& qlambda =
+      const auto qlambda =
         [&](const Coordinate& qpGlobal, const Scalar weight)
         {
           Scalar numericalSolutionAtQP = d[elemIdxSLE];