Implement iteration over nodes in DFT order; expose it to Cython

[d-kad]

Introduced a new nested class, GameRep::Nodes, which acts as a standard C++ forward iterator.

• This iterator performs a depth-first, pre-order traversal of the game tree, starting from the root.
• The GameRep class is now directly iterable, through the addition of: begin() and end() methods
• const overloads for both methods to ensure const-correctness in functions like IsPerfectRecall().

Closes #530

[d-kad]

@tturocy I implemented node traversal in operator++ with a single children iterator on stack using exception catching mechanism (mirroring your Python code snippet for the PR-checker)

[tturocy]

As this looks to be converging well towards a solution please do squash on the next iteration and give a good candidate commit message.

[tturocy]

Merged as 89cbddc with some minor adjustments.