add missing specialization of method axpy for FieldVectors

[[Imported from SVN: r3649]]

dune/solvers/common/staticmatrixtools.hh

@@ -88,6 +88,11 @@ class StaticMatrix
                 x.diagonal()[i] += a * y[i][i];
         }
 
+        template <typename FieldType, int n, typename Scalar>
+        static void axpy(Dune::FieldVector<FieldType,n>& x, const Scalar a, const Dune::FieldVector<FieldType,n>& y)
+        {
+            x.axpy(a, y);
+        }
 
 
         // add transformed matrix X = A^T*Y*B ******************************************************