Fix and improve plotting pairwise average fst

[alimanfoo]

• Resolves #567.
• Removes warnings associated with pandas chained assignments.
• Dynamically sizes plot based on number of cohorts.
• Adds a title to the plot.
• Disable colouring by default if an annotation is requested in the upper triangle (as the table then contains a mix of two different types of values and the colouring becomes confusing).
• Code simplification.

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[alimanfoo]

Applies the fix from #540 to resolve the error raised by @KellyLBennett in #567. Here's the fixed plot:

[alimanfoo]

Thanks @leehart and @ahernank