Implement postorder iterator for nodes range.

[tturocy]

This extends the Nodes range iterator to have two modes, preorder (the existing one), and
postorder (added).

These are runtime-selectable via the TraversalOrder parameter. The default is preorder (so existing code still works).

[tturocy]

@d-kad @rahulsavani As it turned out, as I was unpicking some matters in MixedBehaviorProfile to work on another issue, I realised that it would be a very good idea to implement postorder iteration on nodes there as well. So I have proposed an extension to the Nodes range which allows both preorder and postorder iteration. Tests pass so the refactored preorder seems to be correct. Postorder is not tested however.

[d-kad]

To test it, I updated the game.h file in the current subgame root finder branch and replaced the suboptimal vector-reversal approach in FindSubgameRoots with a call to the new iterator. All tests in the subgames suite passed.

[tturocy]

Absent directly having tests, I'm going to take that as good enough for present purposes!