replace append with set_columns

[petrelharp]

This should be faster!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.83%. Comparing base (e2b1e75) to head (b0a37ec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #402      +/-   ##
==========================================
- Coverage   96.84%   96.83%   -0.02%     
==========================================
  Files           8        8              
  Lines         792      789       -3     
  Branches      149      146       -3     
==========================================
- Hits          767      764       -3     
  Misses         15       15              
  Partials       10       10              

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