Checking for duplicated node names and missing node names is in line
with the schema. Previously some calls to `export v2` would be ok with
missing node names (e.g. see the updated tests in `minify-output.t`) but
any usage with metadata would result in an uncaught error.

Multiple trees ("subtrees") have been available in Auspice since late
2021¹ and part of the associated schema since early 2022². Despite this
there was no way to produce such datasets within Augur itself, and
despite the schema changes the associated `augur validate` command was
never updated to allow them.

This commit adds multi-tree inputs to `augur export v2` as well as
allowing them to validate with our associated validation commands.

¹ <nextstrain/auspice#1442>
² <#851>

Needed for pipelines which will produce multiple trees via `augur
refine` and then supply these trees to `augur export v2`