Update combat.py

[tjuHaoXiaotian]

Fix some bugs:

    1. In get_agent_obs(), the local variable '_type' was mistakenly written as 'type'.
    2. In get_agent_obs(), the x-coordinates and y-coordinates of the entites (whether the entity is 'agent' or 'opponent') within each agent's view range were incorrectly set to the agent's position.
    3. self.agent_prev_pos and self.opp_prev_pos are not updated, but in __update_agent_view() and __update_opp_view(), these previous positions are cleared. Thus the movement records are incorrect.
    4. The self._agent_cool and self._opp_cool are not updated (i.e., the agents and opponents could execute 'attack' action at every step if within range).
        updates:
          (1) update the is_fireable() function to check if the attack agents have been cooled down. 
          (2) add self._agent_cool_step and self._opp_cool_step to track the cooling down of the agents and opponents.
    5. In the opps_action() function, the dead opponent could still call reduce_distance_move() function to sample a 'move' action, which will raise an 'no where to move' exception when the position of the dead opponent is the same with any of the agents.
    6. Update the get_agent_obs(): only return the local view of observation when the agent is alive, otherwise return a zero vector.

Add 2 new functions to support CTDE

    7. Add get_state() to support CTDE (centralized training with decentralized execution).

[koulanurag]

Thanks for these changes. I will probably merge it by the end of the month.

Thanks again!

[koulanurag]

Overall looks good! Thanks for the changes.

I can merge it once you resolve the "_render" comment.

[koulanurag]

All looks good. But, Let's write some more exhaustive test cases. You may use this as a reference.