Issue 517: adding GameRep::GetPlays for Node, Infoset, and Action; exposing it to Cython

[d-kad]

Currently work in progress:

@tturocy I tried to extend Game.get_plays to cover the cases of Infoset and Action.
This Cython code does not compile after introducing new GetPlays overloads.
When there is a single overload for Node alone in commit b767236, it compiles.

Could you please help?

[tturocy]

When we're adding/changing something public, we should also have the following:

• Appropriate entry in ChangeLog
• Add function to the documentation index (doc/pygambit.api.rst) in appropriate location
• Consider adding a user guide section (doc/pygambit.user.rst) which illustrates the use/usefulness of the functions.

[tturocy]

Merged at 7df3d34.