Merge branch 'issue/return-reference-to-local-coords-grid-function' into 'master'

Do not return a reference to a local variable in CoordsGridFunction

See merge request amdis/amdis!195

amdis/gridfunctions/CoordsGridFunction.hpp

@@ -44,7 +44,7 @@ namespace AMDiS
       struct Derivative
       {
         template <class T, int N>
-        Dune::DiagonalMatrix<T, N> const& operator()(Dune::FieldVector<T, N> const& /*x*/) const
+        Dune::DiagonalMatrix<T, N> operator()(Dune::FieldVector<T, N> const& /*x*/) const
         {
           return Dune::DiagonalMatrix<T,N>{T(1)};
         }