Revisit operations when grouped

[josevalim]

Ideally we will have a separate test file for testing how each of them works when grouped, removing all of .group calls from data_frame_test.exs.

Group handling

• [x] group_by
• [x] ungroup
• [x] summarize

Behaves different under group:

• [ ] arrange (in dplyr the group behaviour is opt-in)
• [ ] collect (document that collecting a grouped data frame should return a grouped data frame)
• [ ] distinct
• [ ] head
• [ ] mutate
• [ ] rename (rename grouped columns)
• [ ] rename_with (rename grouped columns)
• [ ] sample (sample 5 of each group)
• [ ] select (raise if dropping grouped columns)
• [ ] slice
• [ ] tail
• [ ] take

No changes under group

• [ ] filter
• [ ] n_columns
• [ ] n_rows
• [ ] pull
• [ ] shape
• [ ] to_* (a small test for some of them is enough)

To discuss

• [ ] dummies
• [ ] pivot_longer
• [ ] pitot_wider
• [ ] concat_rows (what if one side has the grouping and the other does not?)
• [ ] join (what if one side has the grouping and the other does not?)