Improve clonotype network plot with plotAtlas

[grst]

Networkx now offers an "Atlas" example which could be useful for making nicer, packed plots that the current approach using "squarify". Not sure how it handles node sizes and probably the performance is not as good if there are many clonotypes, but it's definitely worth a look:

https://networkx.org/documentation/stable/auto_examples/graphviz_layout/plot_atlas.html#sphx-glr-auto-examples-graphviz-layout-plot-atlas-py

[grst]

Another alternative: https://github.com/paulbrodersen/netgraph