Add data values to plot_hawkes_kernel_norms (annot in seaboard.heatmap)

[MarcosCarreira]

I really like the way plot_hawkes_kernel_norms formats the labels (the arrows and the subscript formatting), and I got used to the From Columns To Rows format. From what I see, it is running something very similar to seaborn.heatmap with options {center=0, cmap='RdBu'}; it would be interesting to also allow the annot and fmt options (e.g. {annot=True, fmt=".3f"}).