Add strategy to strategic game

[tturocy]

The C++ library permits adding a strategy to a game in strategic form, via the NewStrategy() method on GamePlayer. This issue proposes exposing this operation in Python.

• Implement an add() method on the strategies collection of a player, operating similarly to the add() method on players of a game.
• Issue: this operation only makes sense on a strategic game. If the game is an extensive game, detect, and raise an appropriate error.
• Document the existence and operation of this function.
• Add entries to the test suite to exercise this function.