ENH: Implement pure strategy node reachability check

[d-kad]

Description.

This pull request introduces a new method, GameNode::IsStrategyReachable(), to determine if a given node in an extensive tree form can be reached by any pure strategy profile. A node is reachable if there exists at least one pure strategy profile where the resulting path of play passes through that node. A pure strategy requires the player to choose the same action at every member node of that information set. In games with absent-mindedness, a path of play may visit the same information set multiple times, making some of the ancestors of its member nodes unreachable.

The existing GameTreeRep::BuildInfosetParents() method already handles the logic for absent-minded revisiting information sets. This traversal will be leveraged to collect the set of reachable decision nodes.

Implementation Summary.
The implementation follows the design discussed in the issue:

1. A new cache member m_reachableNodes is added to GameTreeRep
2. The existing GameTreeRep::BuildInfosetParents() traversal is augmented to populate this set, reusing its logic for handling absent-mindedness.
3. A new C++ method, GameNodeRep::IsStrategyReachable(), queries the cache
4. The functionality is exposed to Python as a new read-only property, pygambit.Node.is_strategy_reachable

Closes #629

[tturocy]

Internally this really needs to be implemented as a set of nodes (or subtrees) which are not reachable. Un-reachability is the exceptional case not the usual case.

[tturocy]

A further thought on these tests. By definition all nodes are reachable in a game that does not have absent mindedness. This could be written as a different test that automatically just checks all nodes are reachable and that none of the information sets are absent-minded (once that feature is done).