Relabel clades numerically, rather than geographically [#12]

[genehack]

Also update nextclade build README to give mapping back to literature genotypes.

Closes #12

[joverlee521]

Seems reasonable with a quick look at the updated Nextclade dataset. I left some non-blocking questions, but LGTM.

[joverlee521]

I don't remember if we discussed in the meeting whether we want multiple clade systems in the Nextclade dataset, i.e. should we have these numeric clades as the default, but still include the geographical genotype labels as a separate color-by?

[genehack]

I don't remember if we discussed in the meeting whether we want multiple clade systems in the Nextclade dataset, i.e. should we have these numeric clades as the default, but still include the geographical genotype labels as a separate color-by?

The impression that I took away from the feedback in that meeting was that we wanted to de-emphasize the geographically oriented names, so I would lean away from allowing color-by for them.