Add utility function markBoundaryDOfs

If we have `markBoundaryPatchDofs()` there should also be
`markBoundaryDofs()` to avoid having to construct a `BoundaryPatch`
in the trivial cases.

dune/fufem/functiontools/boundarydofs.hh

@@ -50,6 +50,39 @@ void markBoundaryPatchDofs(const BoundaryPatch<GridView>& boundaryPatch,
     });
 }
 
+/**
+ * \brief For a given basis determine all degrees of freedom on the boundary.
+ *
+ * Set each entry of the vector to true, that corresponds to a DOF
+ * associated to an intersection (or a subentity) contained in the
+ * boundary. If the passed vector does not implement
+ * the VectorBackend concept, it is automatically wrapped in a
+ * Dune::Functions::ISTLVectorBackend.
+ *
+ * \param basis Mark DOFS of this basis
+ * \param vector A bit vector corresponding to the basis DOFs
+ */
+template <class Basis, class Vector>
+void markBoundaryDofs(const Basis& basis,
+                           Vector& vector)
+{
+    auto&& toVectorBackend = [&](auto& v) -> decltype(auto) {
+        if constexpr (Dune::models<Dune::Functions::Concept::VectorBackend<Basis>, decltype(v)>()) {
+            return v;
+        } else {
+            return Dune::Functions::istlVectorBackend(v);
+        }
+    };
+
+    auto&& vectorBackend = toVectorBackend(vector);
+    vectorBackend.resize(basis);
+    vectorBackend = false;
+    Dune::Functions::forEachBoundaryDOF(basis, [&] (auto&& index) {
+        vectorBackend[index] = true;
+    });
+}
+
+
 } // namespace Dune::Fufem