Introduce SumFunctionalConstrainedLinearization

Previously, the brittle-fracture program used a taylor-made linearization called ElasticEnergyConstrainedLinearization. However, there wasn't actually anything elasticity-specific in the code, and therefore this MR replaces said class by a generic SumFunctionalConstrainedLinearization.

dune/fracture-phasefields/sumfunctionalconstrainedlinearization.hh

@@ -49,7 +49,7 @@ public:
     // Compute second derivatives of the functionals
     hessian_ = std::get<0>(addends_).hessian();
 
-    Hybrid::forEach(Hybrid::integralRange(std::integral_constant<int, 1>(),Hybrid::size(addends_)), [&](auto&& i) {
+    Hybrid::forEach(Hybrid::integralRange(std::integral_constant<int, 1>(),Hybrid::size(addends_)), [&](auto i) {
       hessian_ += std::get<i>(addends_).hessian();
     });