Modify Fst function to have option for empty upper triangle.

[KellyLBennett]

It would be good to add functionality to the annotation parameter of the plot_pairwise_fst function to have a blank upper triangle on request.

I believe this could be done adding a mask when invoked.

fig_df = fig_df.where(np.tril(np.ones(fig_df.shape)).astype(np.bool_))

[NobleCoder69]

/assign