#12 Optimisation that avoid building strategies for loosing games

[Nicolas D' Ippolito]

There is an optimisation to be made in every game solver. We could avoid building the strategy when the initial state of the game is not winning for the controller. This feature requires to add the set of initial states to the game structure, and extending the game interface to be able to answer if a game is winning or not. This applies to all game solvers -- e.g. GRGameSolver, ConcurrencyGRGameSolver.