(WIP) feature: add observers() method in environments

[YuriCat]

Each environment should determine the observers at each turn.

Next problem: How do you use the observation argument, and is it necessary?

[ikki407]

It's simple (I think observation argument is not necessary.) I expect that users can understand observers() function than before.

Question: If a user defined a player in both turn_players and observers, should HandyRL detect it or raise an error?
Answer: no problem

[ikki407]

Should we add this in the document?
https://github.com/DeNA/HandyRL/blob/master/docs/custom_environment.md