ENH: Implement concept of `GameSubgameRep` in C++/`GameSubgame` in Python

[tturocy]

Analogous to the way we derive GameStrategyRep from extensive games, we will now add a class in C++ GameSubgameRep, which will implement the notion of a subgame.

The cacheing and invalidation mechanism for this will be the same as how we manage strategies on changes to the game.

The following functions relating to subgames will be defined on GameRep:

• GetParentSubgame(const GameSubgame &)
• GetChildSubgames(const GameSubgame &)
• GetRootSubgame()
• GetTerminalSubgames()
• GetInfosets(const GameSubgame &, const GamePlayer &)

These will also be added to Python in the straightforward way, with appropriate tests.

[tturocy]

@d-kad @rahulsavani I've set up a branch dev_subgame_objects linked to this issue which has set up the necessary boilerplate for defining these objects in C++ and wrapping them in Python.