Implement, test, document node properties

[tturocy]

There is a boilerplate Node class in the Cython interface, wrapping the C++ class GameNode. The following operations should be implemented:

• Read-only property 'infoset' (corresponds to C++ GetInfoset())
• Read-only property 'player' (corresponds to C++ GetPlayer())
• Read-only property 'parent' (C++ GetParent())
• Read-only property 'prior_action' (C++ GetPriorAction())
• Read-only property 'next_action' (C++ GetNextAction())
• Read-only property 'prior_sibling' (C++ GetPriorSibling())
• Read-only property 'next_sibling' (C++ GetNextSibling())
• Read-only property 'is_terminal' (C++ IsTerminal())
• Read-only property 'outcome' (C++ GetOutcome(). SetOutcome() also exists but will be handed in a separate issue.)
• Member function is_successor_of (C++ IsSuccessorOf())
• Member function is_subgame_root (C++ IsSubgameRoot())

Most of these properties can return players, nodes, etc. which are null. They should be rendered as 'None' in the Python interface.

In addition, these and the other existing operations should be documented, and entries made into the test suite.