fix dirichlet handling of initial guess

The initial guess of the original system is already used in the RHS creation. To stay consistent, i.e. using the solution as initial guess results in immediate convergence, the initial guess for the constraint system has to be set to zero.

src/hyteg/ginkgo/GinkgoDirichletHandling.hpp

@@ -215,13 +215,7 @@ class ZeroRowsDirichletHandler : public DirichletHandlerBase
 
       orig_initial_guess_ = cur_initial_guess;
       z_                  = gko::clone( cur_initial_guess );
-      for ( auto idx : this->bcIndices_ )
-      {
-         if ( auto lidx = to_local_idx( idx, part, rank ); lidx )
-         {
-            z_->get_local()->at( *lidx ) = gko::zero< valueType >();
-         }
-      }
+      z_->fill(gko::zero<valueType >());
       return z_.get();
    }