feat!: Update resulting column names in pivot when pivoting by multiple values

[MarcoGorelli]

Closes #14491

Changes

• In DataFrame.pivot, when specifying multiple values columns, the result would redundantly include the column column in the column names. This has been addressed.

Example

Before:

>>> df = pl.DataFrame(
...     {
...         "name": ["Cady", "Cady", "Karen", "Karen"],
...         "subject": ["maths", "physics", "maths", "physics"],
...         "test_1": [98, 99, 61, 58],
...         "test_2": [100, 100, 60, 60],
...     }
... )
>>> df.pivot(index='name', columns='subject', values=['test_1', 'test_2'])
shape: (2, 5)
┌───────┬──────────────────────┬────────────────────────┬──────────────────────┬────────────────────────┐
│ name  ┆ test_1_subject_maths ┆ test_1_subject_physics ┆ test_2_subject_maths ┆ test_2_subject_physics │
│ ---   ┆ ---                  ┆ ---                    ┆ ---                  ┆ ---                    │
│ str   ┆ i64                  ┆ i64                    ┆ i64                  ┆ i64                    │
╞═══════╪══════════════════════╪════════════════════════╪══════════════════════╪════════════════════════╡
│ Cady  ┆ 98                   ┆ 99                     ┆ 100                  ┆ 100                    │
│ Karen ┆ 61                   ┆ 58                     ┆ 60                   ┆ 60                     │
└───────┴──────────────────────┴────────────────────────┴──────────────────────┴────────────────────────┘

After:

>>> df = pl.DataFrame(
...     {
...         "name": ["Cady", "Cady", "Karen", "Karen"],
...         "subject": ["maths", "physics", "maths", "physics"],
...         "test_1": [98, 99, 61, 58],
...         "test_2": [100, 100, 60, 60],
...     }
... )
>>> df.pivot(index='name', columns='subject', values=['test_1', 'test_2'])
┌───────┬──────────────┬────────────────┬──────────────┬────────────────┐
│ name  ┆ test_1_maths ┆ test_1_physics ┆ test_2_maths ┆ test_2_physics │
│ ---   ┆ ---          ┆ ---            ┆ ---          ┆ ---            │
│ str   ┆ i64          ┆ i64            ┆ i64          ┆ i64            │
╞═══════╪══════════════╪════════════════╪══════════════╪════════════════╡
│ Cady  ┆ 98           ┆ 99             ┆ 100          ┆ 100            │
│ Karen ┆ 61           ┆ 58             ┆ 60           ┆ 60             │
└───────┴──────────────┴────────────────┴──────────────┴────────────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.44%. Comparing base (462bc8b) to head (d336eb6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16439      +/-   ##
==========================================
+ Coverage   81.43%   81.44%   +0.01%     
==========================================
  Files        1409     1409              
  Lines      184678   184678              
  Branches     2971     2971              
==========================================
+ Hits       150396   150416      +20     
+ Misses      33762    33742      -20     
  Partials      520      520              

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