[ENH]: Require unique labels for game objects (within appropriate scope)

[tturocy]

In a previous release we instituted a future warning that Gambit would require unique, nonempty labels for elements of collections in games. In 16.7 we will now enforce this in preparation for the new game operations in 17.0.

To do:

• Implement uniqueness checks in C++ and raise appropriate exceptions:
  • Players
  • Actions
  • Strategies
  • Outcomes
  • Nodes - we want node labeling to be optional
  • Infosets? - we want infoset labeling to be optional
• Update all example game files to have unique, nonempty labels where required
• Confirm that labels are assigned on creation as appropriate in C++
• Check test suite exercises label checks correctly for all types of elements - including when creating a new game
• Update documentation as appropriate