Implement operator() for global nonlinearities

dune/tectonic/globalnonlinearity.hh

@@ -18,6 +18,15 @@ class GlobalNonlinearity {
   typedef VectorTypeTEMPLATE VectorType;
   typedef MatrixTypeTEMPLATE MatrixType;
 
+  double operator()(VectorType const &x) const {
+    double tmp = 0;
+    for (size_t i = 0; i < x.size(); ++i) {
+      auto const res = restriction(i);
+      tmp += (*res)(x[i]);
+    }
+    return tmp;
+  }
+
   /*
     Return a restriction of the outer function to the i'th node.
   */