Bug fix and improve MCTS and others

• Fixed error with mean and std
• Improved MCTS.
  1. Fixed a bug of starting with start_node;
  2. Fixed the miscalculation of mean and std;
  3. Added timeout to prevent infinite-loop.
  4. Changed output format in mcts.py
  5. updated and tested mcts.py
• Refactoring (env -> worlds)
• Change AP ('before_but_close_to_stop_region' --> 'close_to_stop_region')