Return target columns in Map

[juliohm]

@eliascarv I remember we opted to return all columns of the original table to mimic DataFrames.jl's transform behavior. Is there any other reason? A more ergonomic behavior is proposed in this PR that only returns the new (target) columns in the result.

[eliascarv]

@juliohm There was no other reason. I remember that mimicking the behavior of DataFrames.jl was the only reason.

Therefore, I think we can continue with this change.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (52a900d) to head (be406eb).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
- Coverage   98.34%   98.33%   -0.02%     
==========================================
  Files          50       50              
  Lines        1450     1440      -10     
==========================================
- Hits         1426     1416      -10     
  Misses         24       24              

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