Create nextclade dataset

[victorlin]

from @trvrb on Slack

We'd want a Nextclade dataset that labels EBOV genomes according to outbreak. Each outbreak is it's own clade. I believe this will require a new clades.tsv, but this shouldn't be too difficult. We can target artic-network's TSV.

This would provide the default coloring for ebola/all-outbreaks/.