Fix: No descent if the gradient is too small

At least for the tangential descent, a gradient of magnitude 1e-16
causes problems. Test included

dune/tectonic/minimisation.hh

@@ -104,7 +104,7 @@ void minimise(Functional const &J, typename Functional::SmallVector &x,
     SmallVector descDir;
     bool const linesearchp = J.descentDirection(x, descDir);
 
-    if (descDir == SmallVector(0.0))
+    if (descDir.two_norm() < 1e-14) // TODO: Make controllable
       return;
 
     if (linesearchp) {

src/test-gradient-sample-steep.cc

@@ -20,7 +20,7 @@ int main() {
   typedef Functional::SmallVector SmallVector;
 
   SmallMatrix const A = { { 1, 0 }, { 0, 1 } };
-  SmallVector const b = { 1, 2 };
+  SmallVector const b = { 1, 2.5 };
 
   auto const f = Dune::make_shared<Dune::SampleFunction<2> const>();
   auto const phi = Dune::make_shared<Functional::NonlinearityType const>(f);

src/test-gradient-sample-steep2.cc

@@ -29,7 +29,7 @@ int main() {
   double ret1;
   {
     SmallVector start = { 0, 1 };
-    ret1 = functionTester(J, start, 1);
+    ret1 = functionTester(J, start, 2);
   }
 
   double ret2;

src/test-gradient-sample-verysteep.cc

@@ -29,7 +29,7 @@ int main() {
   double ret1;
   {
     SmallVector start = { 0, 1 };
-    ret1 = functionTester(J, start, 1);
+    ret1 = functionTester(J, start, 2);
   }
 
   double ret2;