[concept] Add concept for a VectorBackend

dune/functions/functionspacebases/concepts.hh

@@ -250,6 +250,19 @@ struct GlobalBasis
   );
 };
 
+
+
+// Concept for a VectorBackend
+template<class GlobalBasis>
+struct VectorBackend
+{
+  template<class V>
+  auto require(const V& v) -> decltype(
+    const_cast<V&>(v).resize(std::declval<const GlobalBasis&>()),
+    v[std::declval<typename GlobalBasis::MultiIndex>()]
+  );
+};
+
 } // namespace Dune::Functions::Concept
 } // namespace Dune::Functions
 } // namespace Dune