Add major clade assignments

[kimandrews]

Context

8 major clades of rabies seem to be generally accepted in the literature, and it would be helpful to visualize these on the phylogeny.

These clades are defined in Troupin et al. 2016 and include:

Cosmopolitan
Arctic-related
Africa-2
Africa-3
Asian
Indian sub.
Bats
RAC-SK

Possible solution

Use augur clades to define major clades.