[assembler.system] allow member access for derived classes

dune/gdt/assembler/system.hh

@@ -201,8 +201,7 @@ public:
     this->walk(partitioning);
   }
 
-
-private:
+protected:
   const DS::PerThreadValue<const TestSpaceType> test_space_;
   const DS::PerThreadValue<const AnsatzSpaceType> ansatz_space_;
 }; // class SystemAssembler