[Cleanup] Use Axy()

dune/tectonic/mydirectionalconvexfunction.hh

@@ -4,6 +4,7 @@
 #ifndef MY_DIRECTIONAL_CONVEX_FUNCTION_HH
 #define MY_DIRECTIONAL_CONVEX_FUNCTION_HH
 
+#include <dune/fufem/arithmetic.hh>
 #include <dune/fufem/interval.hh>
 
 /*
@@ -16,9 +17,7 @@
 
 template <class MatrixType, class VectorType>
 double computeDirectionalA(MatrixType const &A, VectorType const &v) {
-  VectorType tmp(v.size());
-  A.mv(v, tmp);
-  return tmp * v;
+  return Arithmetic::Axy(A, v, v);
 }
 
 template <class MatrixType, class VectorType>