Fix BUG:Getting strategic contingency for game in extensive form

The crash reason of getting outcome in extensive-form game is that in
C++ part(TreePureStrategyProfileRep) doesn't implement the GetOutcome
function(just throw UndefinedException).

The current solution now is creating a Cython-level TreeGameOutcome
which wrapping the C++ TreePureStrategyProfileRep class stands in for
an outcome in the extensive-form game.