[Algorit] Initial iterate: 0

If we use the old velocity, it can happen that we hit the kernel of
the singular norm component

src/timestepping.cc

@@ -139,7 +139,7 @@ void Newmark<VectorType, MatrixType, FunctionType, dim>::setup(
   Arithmetic::addProduct(problem_AB, 2.0 / tau, B);
 
   // v_old makes a good initial iterate; we could use anything, though
-  problem_iterate = v_old;
+  problem_iterate = 0.0;
 
   for (size_t i = 0; i < dirichletNodes.size(); ++i)
     switch (dirichletNodes[i].count()) {