export: Allow --node-data to be repeated to build up a file list

[tsibley]

Description of proposed changes

…instead of each repetition overriding the last values. While there is some theoretical utility in being able to override one list of node data files with another in a subsequent option, I think it's more likely that repetition is intended to built up a single list. I was surprised, at least.

See also https://discussion.nextstrain.org/t/1194/4.

Testing

• New and updated tests pass locally
• CI passes

Checklist

• Add a message in CHANGES.md summarizing the changes in this PR. Keep headers and formatting consistent with the rest of the file.
• Note that this would be a backwards incompatible change in the changelog.

[tsibley]

ISTR discussing this behaviour (or similar) before but can't recall specifics. In any case, I think the behaviour in this PR is more favorable, but it's definitely up for discussion. Are there many (any?) cases where we really do want the overriding behaviour?

[codecov]

Codecov Report

Base: 61.68% // Head: 59.68% // Decreases project coverage by -1.99% ⚠️

Coverage data is based on head (14f48ec) compared to base (862bbb6).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 14f48ec differs from pull request most recent head de8147b. Consider uploading reports for the commit de8147b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1010      +/-   ##
==========================================
- Coverage   61.68%   59.68%   -2.00%     
==========================================
  Files          52       53       +1     
  Lines        6287     6323      +36     
  Branches     1583     1587       +4     
==========================================
- Hits         3878     3774     -104     
- Misses       2138     2286     +148     
+ Partials      271      263       -8     

Impacted Files	Coverage Δ
augur/argparse_.py	82.60% <100.00%> (+3.66%)	⬆️
augur/export_v1.py	74.46% <100.00%> (+0.10%)	⬆️
augur/export_v2.py	68.93% <100.00%> (-0.11%)	⬇️
augur/io/vcf.py	42.50% <0.00%> (-43.75%)	⬇️
augur/translate.py	38.67% <0.00%> (-35.94%)	⬇️
augur/utils.py	64.25% <0.00%> (-7.36%)	⬇️
augur/dates.py	89.85% <0.00%> (-3.01%)	⬇️
augur/filter.py	95.82% <0.00%> (-0.01%)	⬇️
augur/util_support/node_data_file.py	100.00% <0.00%> (ø)
... and 4 more

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[jameshadfield]

This seems good to go from my perspective. It's not something I've come across but I agree that the interpretation of repeated --node-data should be the union of files, not simply the last one(s), and thus think it should be merged.

[tsibley]

Updated the changelog in f48f196 and then rebased onto latest master to resolve conflicts before merge.