[spaces.interface] add default_space_backend

dune/gdt/spaces/interface.hh

@@ -39,6 +39,16 @@ enum class ChooseSpaceBackend
 }; // enum class ChooseSpaceBackend
 
 
+static constexpr ChooseSpaceBackend default_space_backend =
+#if HAVE_DUNE_FEM
+    ChooseSpaceBackend::fem;
+#elif HAVE_DUNE_PDELAB
+    ChooseSpaceBackend::pdelab;
+#else
+    ChooseSpaceBackend::gdt;
+#endif
+
+
 template <ChooseSpaceBackend type>
 struct ChooseGridPartView;