Add player labels for normal form game visuals

[edwardchalstrey1]

Issues closed by this PR

• Closes Add player labels to normal form game visual #574

Description of the changes in this PR

This PR adds player labels to the printout of normal form games:

• Label should always be centred, based on the number of columns (no. of player strategies)
• Updates the test to check labels exist

How to review this PR

• Check the player labels look right in tutorial 1 once rtd renders it (screenshot above)

[edwardchalstrey1]

How it looks in the docs:

[tturocy]

Immediate queries:

• What's this doing with games with more than two players?
• What about the LaTeX output? Should we add the same feature?

At the risk of scope creep there's a bunch of different ways we might output tables, and one of the problems we have right now is code duplication (as we discovered in the earlier PR #757). LaTeX and HTML tables have basically the same iteration just different output of symbols.

Does it make sense to refactor to support possibly different outputs? For example markdown tables could be another possibility - not that we do that right now but it would become very easy to add with the right abstraction.

But it might also just be too much?