message: add comments to IR
diff: @@ -92,9 +92,16 @@ void Ir<ValueType>::apply_impl(const LinOp *b, LinOp *x) const
  break;
  }
  + // Use the inner solver to solve
  + // A * inner_solution = residual
  + // with residual as initial guess.
  inner_solution->copy_from(residual.get());
  solver_->apply(lend(residual), lend(inner_solution));
  +
  + // x = x + inner_solution
  dense_x->add_scaled(lend(one_op), lend(inner_solution));
  +
  + // residual = b - A * x
  residual->copy_from(dense_b);
  system_matrix_->apply(lend(neg_one_op), dense_x, lend(one_op),
  lend(residual));
  