Tidy up MixedStrategyProfile implementation

Create operations IsRestriction() and Unrestrict() on games, which provides
access to the unrestricted version of the gmae.

Create an outline of Unrestrict() operation on MixedStrategyProfile, which
will need to be filled in with mapping the probabilities from the
profile on the restricted game to one on the unrestricted game.


Next steps: The MixedStrategyProfile<T>::Unrestrict() operation needs to be filled in, to map the probabilities from the profile on the restricted game, to the equivalent profile on the unrestricted game.  (Probabilities of strategies not in the restricted game are zero; this is already taken care of by initializing the profile to zero which is already done in this implementation.)

(The commits added 3 July take care of this.)