Extend augur import with auspice mode that imports from auspice.json

[corneliusroemer]

Context

It's a common problem that one wants to decompose an auspice.json file into newick tree and metadata.

It's something many people have written ad hoc scripts for, but there's no augur command for this purpose.

Description

Add a type auspice to augur import besides the only type so far: beast.

Output could be:

• newick tree
• metadata.tsv
• branch_data.jsons

It might be nice to also publicly expose a Python function in the public augur Python API to allow an import of auspice.json directly into a Python datastructure, to avoid having to call the CLI when one just wants to stay in Python.

This was previously requested by a user and discussed in: #438

I think many Nextstrain people have their own scripts, e.g. @huddlej links to one in the issue above. I also have one, Richard too, for sure.