Add sumenergy.hh, this class assembles the a sum of energies for a single...

Add sumenergy.hh, this class assembles the a sum of energies for a single element by summing up the energies of each GFE::LocalEnergy.

This class works similarly to the class Dune::Elasticity::SumEnergy, where Dune::Elasticity::SumEnergy extends
Dune::Elasticity::LocalEnergy and Dune::GFE::SumEnergy extends Dune::GFE::LocalEnergy.

dune/gfe/sumenergy.hh

+#ifndef DUNE_GFE_SUMENERGY_HH
+#define DUNE_GFE_SUMENERGY_HH
+
+#include <vector>
+
+#include <dune/gfe/localenergy.hh>
+#include <dune/gfe/mixedlocalgeodesicfestiffness.hh>
+
+namespace Dune::GFE {
+
+  /**
+    \brief Assembles the a sum of energies for a single element by summing up the energies of each GFE::LocalEnergy.
+
+           This class works similarly to the class Dune::Elasticity::SumEnergy, where Dune::Elasticity::SumEnergy extends
+           Dune::Elasticity::LocalEnergy and Dune::GFE::SumEnergy extends Dune::GFE::LocalEnergy.
+  */
+
+template<class Basis, class... TargetSpaces>
+class SumEnergy
+  : public Dune::GFE::LocalEnergy<Basis, TargetSpaces...>,
+    public MixedLocalGeodesicFEStiffness<Basis, TargetSpaces...>
+    //Inheriting from MixedLocalGeodesicFEStiffness is hack, and will be replaced eventually; once MixedLocalGFEADOLCStiffness
+    //will be removed and its functionality will be included in LocalGeodesicFEADOLCStiffness this is not needed anymore!
+
+{
+
+  using LocalView = typename Basis::LocalView;
+  using GridView = typename LocalView::GridView;
+  using DT = typename GridView::Grid::ctype;
+  using RT = typename Dune::GFE::LocalEnergy<Basis,TargetSpaces...>::RT;
+
+public:
+  
+  /** \brief Default Constructor*/
+  SumEnergy()
+  {}
+
+  void addLocalEnergy(std::shared_ptr<GFE::LocalEnergy<Basis,TargetSpaces...>> localEnergy)
+  {
+    localEnergies_.push_back(localEnergy);
+  }
+
+  RT energy(const typename Basis::LocalView& localView,
+       const std::vector<TargetSpaces>&... localSolution) const
+  {
+    RT sum = 0.;
+    for ( const auto& localEnergy : localEnergies_ )
+      sum += localEnergy->energy( localView, localSolution... );
+
+    return sum;
+  }
+
+private:
+  // vector containing pointers to the local energies
+  std::vector<std::shared_ptr<GFE::LocalEnergy<Basis, TargetSpaces...>>> localEnergies_;
+};
+
+}  // namespace Dune::GFE
+
+#endif   //#ifndef DUNE_GFE_SUMENERGY_HH