ENH: Allow use of nextalign in augur translate

[corneliusroemer]

Context

It'd be good if augur translate could interface with nextalign.

We have an explicit_translation.py script that does the job, but it'd be nice to integrate into translate.

The script can be found here for example: https://github.com/neherlab/ncov-simple/blob/master/scripts/explicit_translation.py

Right now augur translate asks for ancestrally reconstructed node data for internal nodes. The explicit translation script starts with the translations and infers internal nodes.