message: Fix GmresMixed core problem
diff: @@ -443,7 +443,7 @@ void GmresMixed<ValueType>::apply_impl(const LinOp *b, LinOp *x) const
  // Solve upper triangular.
  // y = hessenberg \ residual_norm_collection
  - get_preconditioner()->apply(before_preconditioner.get(),
  + this->get_preconditioner()->apply(before_preconditioner.get(),
  after_preconditioner.get());
  dense_x->add_scaled(one_op.get(), after_preconditioner.get());
  // Solve x
  @@ -470,7 +470,7 @@ void GmresMixed<ValueType>::apply_impl(const LinOp *b, LinOp *x) const
  #endif
  }
  - get_preconditioner()->apply(next_krylov_basis.get(),
  + this->get_preconditioner()->apply(next_krylov_basis.get(),
  preconditioned_vector.get());
  // preconditioned_vector = get_preconditioner() *
  // next_krylov_basis
  @@ -566,7 +566,7 @@ void GmresMixed<ValueType>::apply_impl(const LinOp *b, LinOp *x) const
  exec->synchronize();
  auto t_aux_4 = std::chrono::steady_clock::now();
  #endif
  - get_preconditioner()->apply(before_preconditioner.get(),
  + this->get_preconditioner()->apply(before_preconditioner.get(),
  after_preconditioner.get());
  dense_x->add_scaled(one_op.get(), after_preconditioner.get());
  #ifdef TIMING_STEPS
  