Support libc++ and reduce minimum required standard to c++20

• Use alternative to std::move_only_function from function2 library
  • head only library
  • provides unique_function
• Issues with using std::move_only_function:
  • requires c++23 with libstdc++
  • not yet implemented by libc++
• Reduce minimum required standard in CMakeLists.txt to c++20
• Add missing lambda parentheses

dune/copasi/model/reduce.hh

@@ -14,6 +14,7 @@
 
 #include <dune/common/float_cmp.hh>
 
+#include <function2/function2.hpp>
 #include <spdlog/spdlog.h>
 
 #include <fmt/color.h>
@@ -39,8 +40,7 @@ reduce(const Basis& basis,
                 const ParameterTree& config,
                 std::shared_ptr<const FunctorFactory<Basis::EntitySet::GridView::dimension>> functor_factory = nullptr)
 {
-  using GridView = typename Basis::EntitySet::GridView;
-  constexpr std::size_t dim = GridView::dimension;
+  constexpr std::size_t dim = Basis::EntitySet::GridView::dimension;
   TRACE_EVENT("dune", "Reduce");
 
   if (basis.entitySet().size(0) == 0) {
@@ -48,7 +48,7 @@ reduce(const Basis& basis,
   }
 
   std::shared_ptr<const ParserContext> parser_context;
-  auto functor_factory_parser = std::dynamic_pointer_cast<const FunctorFactoryParser<GridView::dimension>>(functor_factory);
+  auto functor_factory_parser = std::dynamic_pointer_cast<const FunctorFactoryParser<dim>>(functor_factory);
   if (functor_factory_parser) {
     parser_context = functor_factory_parser->parser_context();
   }
@@ -85,7 +85,7 @@ reduce(const Basis& basis,
 
   std::map<std::string, double> values;
   double factor = 0.;
-  std::vector<std::move_only_function<void() const>> evals;
+  std::vector<fu2::unique_function<void() const>> evals;
   // configure parsers of the evaluation/reduction passes
   for (auto key : config.getSubKeys()) {
     if (config.hasKey(key + ".evaluation.expression")) {
@@ -93,7 +93,7 @@ reduce(const Basis& basis,
       if (not evaluation)
         continue;
       values[key] = 0.;
-      std::move_only_function<double(double, double, double) const> reduction = [](auto init, auto val, auto weight) { return init + val*weight; };
+      fu2::unique_function<double(double, double, double) const> reduction = [](auto init, auto val, auto weight) { return init + val*weight; };
       if (config.hasKey(key + ".reduction.expression")) {
         auto [args, expr] = parser_context->parse_function_expression(config.sub(key).sub("reduction")["expression"]);
         if (args.size() != 3)