Subgraph plotting: enhancement for showing mutations

[hyanwong]

It would be nice for sc2ts.utils.plot_subgraph to be able to plot the number of mutations as a series of blue bars or perpendicular lines, as in Fig 1 of the sc2ts preprint, as an optional replacement for the current dark red numbers. This is only useful for smaller numbers of mutations per edge, but gives a nicer visual feel for the number of mutations on an edge than a number.