Indexing outcome payoffs

[tturocy]

Outcome payoffs can be obtained via indexing using the __getitem__ operator. This takes an integer corresponding to the number the player appears in the list of players in the game.

• Enhancement: Support indexing by player labels (text strings). If there are players with duplicate labels in the game, raise a suitable exception. (That is, it is up to the user to keep player labels unique within a game.)
• Enhancement: Support indexing by player objects. The player object needs to belong to the appropriate game, otherwise an exception should be raised.
• Additional test cases: Ensure that unexpected input raises appropriate exceptions.