Implement operation to copy subgame to independent game.

Refactor a function previously in the solve-by-subgame module,
which copies a subgame rooted at a given node to an independent
game representation.
Implement more flexible interface for game serialization.

Add a new member function to the Game model, Write(), which subsumes
all serialization methods.  Which output format is desired is specified
by an optional parameter, which defaults to 'native'.

Any implementation of game should implement its 'native' output filter,
which should be appropriate to the underlying representation method.
In addition, implementations may support other types of output filters
for conversion purposes.

The Python API has been revised to conform to this convention, and
the Python APi documentation has been updated accordingly.
Update some details on building from the git repository and building the Python API.
Rationalize location of implementation of game tree-specific objects.