[FEAT] Add MultiAggJoiner, refactor AggJoiner

[TheooJ]

Sharing my WIP for issue https://github.com/skrub-data/skrub/issues/810

Adding the MultiAggJoiner also means refactoring the AggJoiner to match the Joiner, in particular:

• Adding a key argument
• Supporting polars in the test suite
• Only accepting a single table as an input
• Performing multiple small checks instead of one big check

Notes:

• For now, the check concerning duplicate column names happens after the aggregation in AggJoiner. Later we / I could move it before aggregation, I didn’t want to touch it for now since it is linked to the _polars.aggregate and _pandas.aggregate functions, let me know how you feel about this
• Should I change the landing page to show the multijoiners, or keep Joiner and AggJoiner ?

Also, thanks @jeromedockes & @Vincent-Maladiere for the discussions !

[TheooJ]

I'm unsure about the default suffixes in MultiAggJoiner. If we add suffixes to the tables to be joined based on their position in the auxiliary table list, it will have a different behavior thanAggJoiner. I think AggJoiner should have the same behavior than MultiAggJoiner when given a list of 1 dataframe

WDYT ?

[TheooJ]

I want to be a bit more consistent with parameter names. In the MultiAggJoiner, I think we want to keep all plurals i.e. aux_tables, keys, main_keys, aux_keys, cols, operations and suffixes.

But in the AggJoiner, there are currently aux_table, key, main_key, aux_key, cols (notice the plural), operation (no plural), suffix.

• aux_table and suffix need to stay singular, but I think we could go either way for the others
• To be consistent with aux_table and suffix (and to contrast with the MultiAggJoiner), we could go singular for all the others, but I think cols and operations are going to be lists in most cases and so plural makes sense for them. We could also have a mix, and just change operation for operations

Wondering how you feel about this @jeromedockes

[jeromedockes]

so IIUC: AggJoiner:

aux_table, key, main_key, aux_key, suffix, cols, operations

MultiAggJoiner:

aux_tables, keys, main_keys, aux_keys, suffixes, cols, operations

is that correct? sounds ok to me

[GaelVaroquaux]

You need a changelog entry for this (adding an entry in doc/CHANGES.rst)