Implement, test, document information set properties

[tturocy]

There exists a boilerplate Cython wrapper of the C++ GameInfoset class. A few needed operations are missing:

• Read-only property 'player', returning the player to which the information set belongs; corresponds to C++ GetPlayer(). (There is also a SetPlayer(); this will be handled in a separate issue.)
• Member function 'precedes', which takes a node; corresponds to C++ Precedes().

In addition, documentation of the existing operations as well as the new ones should be added, and entries made into the test suite to exercise them.