SymmetricMatrix: Add traceMap

The trace map is a vector d, s.t. <d,a> = trace(A) where
A.data = a.

We use the isometry to reduce the computation to the data vectors.

dune/fufem/symmetricmatrix.hh

@@ -48,6 +48,17 @@ public:
     return id;
   }
 
+    /** \brief return the trace map in vector representation
+     *  the returned vector is such that
+     *  <data,a> = trace(A) where A.data = a
+     */
+  static constexpr Data traceMap()
+  {
+    // trace(A) = <Id,A>, therefore return data of Id
+    // use isometry!
+    return identityMatrix().data();
+  }
+
   SymmetricMatrix<T,N>& operator=(const T& s)
   {
     data_ = s;