Provide access to wrapped GlobalBasis in DuneFunctionsBasis<GlobalBasis>

This allows to e.g. use `Dune::Functions::DiscreteGlobalBasisFunction`
with a wrapped basis without storing a separate copy of it.

dune/fufem/functionspacebases/dunefunctionsbasis.hh

@@ -86,6 +86,13 @@ public:
 #endif
   }
 
+  /** \brief Get the wrapper dune-functions GlobalBasis
+   */
+  const DFBasis& functionsBasis() const
+  {
+    return dfBasis_;
+  }
+
 protected:
 
   // The dune-functions basis we are wrapping