Tests for adding player to game

[tturocy]

A player may be added to a game using the add() operation on the players collection on a game. This works for both extensive and strategic games. The following needs to be done:

• Document the existence and behaviour of this operation. For extensive games, it creates a new player with no moves. For strategic games, it creates a new player with one strategy.
• Consider whether any exceptions need to be caught and handled.
• Add entries to the test suite to exercise this operation.