Put tanglegram-creating code into notebook

[hyanwong]

So that plot.py doesn't need to run dendroscope. The intermediate (sorted) files are saved in four tree sequence files in the data/ directory, and I have added them to this commit so that the plots can be created without running the notebooks first. These tsz files contain only a tiny subset of the ARG, so I don't think we should worry about putting them in the repo.

[hyanwong]

There is no effect of this PR on the final output files: It's just tidying up the logic of the pipeline for plot production.