Preserving the line-breaks makes the command much more readable in
Snakemake output¹, which is important since I'm changing this rule right
now.

The \n previously interpreted by Python is now interpreted by `tr`,
which is preferable.

¹ <https://docs.nextstrain.org/en/latest/reference/snakemake-style-guide.html#use-triple-quoted-command-definitions>

This construction reads much clearer and cleaner.

Moves the Nextclade field map directly and more conveniently into the
YAML config instead of referencing a separate TSV file.  Putting the
field map into a separate file seemed to be only for the sake of the
--kv-file (-k) interface provided by `cvstk rename2`, which we're no
longer using here.  For backwards compatibility, configs that reference
a TSV file are still supported and will be handled on-the-fly.

Note that `augur curate` commands currently emit CSV-like TSVs that are
limited to be IANA-like¹ such that parsing them with tsv-utils is most
appropriate, hence the switch from `csvtk cut` to `tsv-select`.

¹ See <nextstrain/augur#1566>.

This construction reads a bit clearer and cleaner.  It's also a good
example of how to use `augur merge`.

The limitation on non-seekable streams means the workflow now uses
additional transient disk space, but it typically shouldn't be an issue.
The way Augur's slow start up time impacts `augur merge` also
contributes to a longer rule execution time, but it should be negligible
in the context of the larger workflow and presumably we'll fix the slow
start up eventually.¹

The output is semantically identical but has some syntactic changes re:
quoting.  It's worth noting that the pre-existing TSV format was _not_
IANA TSV, despite it (still) being treated as such in a few places, but
was (and remains) a CSV-like TSV with some quoted fields (and some
mangled quotes², e.g. the "institution" column for accession KJ556895).
We really need to sort out our TSV formats³, but that's for a larger
project.

¹ <nextstrain/augur#1628>
² <nextstrain/augur#1565>
³ <nextstrain/augur#1566>