Update testing for pygambit integration

[edwardchalstrey1]

At the moment, we just rely on the tests/test_tutorials.py running the tutorial/draw_efg_gambitLayout.ipynb notebook, but it might be better to write more rigorous tests for this.

• Add pytest tests specific to src/core/gambit_layout.py
• Add tests where the gambit object has no player labels (use numbers to denote players "Player 1" etc)
• Ensure multiplier and xshift args cannot cause errors
  • Relataed to Consider normalising pygambit level spacing options #21
• Include an example of a pygambit game loaded from the OpenSpiel library e.g. tiny hanabi

[edwardchalstrey1]

Work has been started on add-openspiel-gambit-example which add-colors also branches off