message: better checkpoint example
diff: @@ -37,14 +37,12 @@ fn run() -> Result<(), Error> {
  let iters = 35;
  let solver = Landweber::new(0.001)?;
  - let res = match Executor::from_checkpoint(".checkpoints/landweber_exec.arg") {
  - Ok(exec) => exec,
  - Err(_) => Executor::new(operator, solver, init_param)
  + let res = Executor::from_checkpoint(".checkpoints/landweber_exec.arg")
  + .unwrap_or(Executor::new(operator, solver, init_param))
  .set_max_iters(iters)
  .checkpoint_dir(".checkpoints")
  .checkpoint_name("landweber_exec")
  - .checkpoint_mode(CheckpointMode::Every(20)),
  - }
  + .checkpoint_mode(CheckpointMode::Every(20))
  .add_logger(ArgminSlogLogger::term())
  .run()?;
  