Improve lineage tree layout for conditions where multiple children arise from a division event.

[lupinthief]

The lineage tree plotting tends to result in overlaps in the graph when more than two children arise from a fragmentation. This looks to be controlled by the generational depth modifier, but may require a more sophisticated layout engine to handle more complex trees. Here's an example.

[quantumjot]

but may require a more sophisticated layout engine

I think this is the crux of it. Laying out planar directed graphs is a tricky problem, so if you have any suggestions, it'd be very welcome!

[lupinthief]

I've been trying (and failing) to get graphviz and pydot to work on Windows, and have yet to put any effort into converting btrack output to dot format. That's where I was going to start. Visualisation not currently a top priority though.