Add code to produce the cophylogeny plot

[hyanwong]

Requires jeromekelleher/sc2ts#112. Here's the current PDF it produces:

cophylogeny.pdf

Note that the SVG is somewhat more interactive, as it allows samples labels to be revealed on hover

[hyanwong]

The "long" plot (for supplementary) needs a bit of minor y-position adjusting, but is not too bad otherwise:

supp_cophylogeny_long.pdf

[hyanwong]

This also requires the files upgma-full-md-30-mm-3-2021-06-30-recinfo-il.ts.tsz, upgma-mds-1000-md-30-mm-3-2022-06-30-recinfo-il.ts.tsz and nextstrain_ncov_gisaid_global_all-time_timetree-2023-01-21.nex to be uploaded into the data dir. The nextstrain .nex file is 290K compressed. Is it worth adding that to the GH repo, as it may be difficult to get exactly the same file from the NextStrain website?