[Cleanup] Add missing #includes

src/timestepping.cc

@@ -3,7 +3,6 @@
 #endif
 
 #include <dune/istl/matrixindexset.hh>
-#include <dune/fufem/arithmetic.hh>
 
 #include "timestepping.hh"
 

src/timestepping/backward_euler.cc

+#include <dune/solvers/common/arithmetic.hh>
+
 template <class Vector, class Matrix, class Function, size_t dim>
 BackwardEuler<Vector, Matrix, Function, dim>::BackwardEuler(
     Matrix const &_A, Matrix const &_M, Matrix const &_C,

src/timestepping/newmark.cc

+#include <dune/solvers/common/arithmetic.hh>
+
 template <class Vector, class Matrix, class Function, size_t dim>
 Newmark<Vector, Matrix, Function, dim>::Newmark(
     Matrix const &_A, Matrix const &_M, Matrix const &_C,