[spaces.constraints] allow access to dirichlet DoFs

dune/gdt/spaces/constraints.hh

@@ -102,6 +102,11 @@ public:
     dirichlet_DoFs_.insert(DoF);
   }
 
+  const std::set<size_t>& dirichlet_DoFs() const
+  {
+    return dirichlet_DoFs_;
+  }
+
   template <class M>
   void apply(Stuff::LA::MatrixInterface<M>& matrix) const
   {