Do not truncate the gradient out-of-place

x/|x| is only a problem if x == 0
just divide by |x|, no matter what non-zero value it is

dune/tectonic/localnonlinearity.hh

@@ -89,7 +89,7 @@ template <int dimension> class LocalNonlinearity {
 
   void addGradient(VectorType const &x, VectorType &y) const {
     double const xnorm = x.two_norm();
-    if (xnorm < 1e-8 or std::isinf(func_->regularity(xnorm))) // TODO
+    if (xnorm == 0 or std::isinf(func_->regularity(xnorm)))
       return;
 
     // left and right differential coincide in this case