Add support for dynamic container of local solvers

This allows to have different local solvers in `NonlinearGSStep`.
The feature existed before but was restricted to statically sized
containers. This patch adds support for dynamically sized ones.

dune/tnnmg/iterationsteps/nonlineargsstep.hh

@@ -15,6 +15,19 @@
 namespace Dune {
 namespace TNNMG {
 
+namespace Impl {
+
+template<class C, class = void>
+struct IsContainer : std::false_type {};
+
+template<class C>
+struct IsContainer<C, std::void_t<decltype(std::declval<C>().size())>> : std::true_type {};
+
+template<class C>
+constexpr bool IsHybridContainer_v = Impl::IsContainer<C>::value or IsTuple<C>::value;
+
+}
+
 
 
 
@@ -50,7 +63,7 @@ private:
  * recursively inside of a localSolver.
  */
 template<class V, class F, class BV, class LS,
-  std::enable_if_t<not IsTuple<LS>::value, int> = 0>
+  std::enable_if_t<not Impl::IsHybridContainer_v<LS>, int> = 0>
 void gaussSeidelLoop(V& x, const F& f, const BV& ignore, const LS& localSolver)
 {
   namespace H = Dune::Hybrid;
@@ -87,17 +100,11 @@ void gaussSeidelLoop(V& x, const F& f, const BV& ignore, const LS& localSolver)
  * recursively inside of a localSolver.
  */
 template<class V, class F, class BV, class LS,
-  std::enable_if_t<IsTuple<LS>::value, int> = 0>
+  std::enable_if_t<Impl::IsHybridContainer_v<LS>, int> = 0>
 void gaussSeidelLoop(V& x, const F& f, const BV& ignore, const LS& localSolvers)
 {
   namespace H = Dune::Hybrid;
 
-  // Clang 3.8 refuses to recognise H::size as constant here, so that
-  // we need to use the decltype(...)::value kludge
-  static_assert(decltype(H::size(x))::value == decltype(H::size(localSolvers))::value,
-                "Size of local solver tuple provided to gaussSeidelLoop does not "
-                "match size of problem.");
-
   auto&& shiftedF = shift(f, x);
 
   H::forEach(H::integralRange(H::size(x)), [&](auto&& i) {