Add Nextstrain clade labels to node metadata

[szhan]

It makes it easier to compare our trees with the Nextstrain tree and replicate some published analysis, e.g. Bloom et al.'s mutational spectra across variants.

These definitions are probably more stable (and clearly far fewer) than the pangolin ones:
https://github.com/nextstrain/ncov/blob/master/defaults/clades.tsv

[jeromekelleher]

Do we have these for all samples? A similar argument against adding metadata that we curate applies as in #223

[szhan]

I'm just hoping to label the root node of the clades, not all the samples.