Rename 'aggregate_function' parameter to 'agg' (API consistency)

[juansolm]

Description

Pivot() has the parameter 'aggregate_function'
https://docs.pola.rs/api/python/stable/reference/dataframe/api/polars.DataFrame.pivot.html

This notation seems to be only in use for pivot:
https://docs.pola.rs/api/python/stable/search.html?q=aggregate_function

Most of the API however uses 'agg'
This is easier to use, clear to understand, and more consistent across the entire API
https://docs.pola.rs/api/python/stable/search.html?q=agg

Current

df.pivot("col", index="ix", aggregate_function="sum")

New

df.pivot("col", index="ix", agg="sum")

[nikaltipar]

I can make a change for that, just for fun. I would probably rename aggregate_function to agg_function, to disambiguate with other options, such as agg_list that exist in other methods

[orlp]

@nikaltipar I appreciate the PR but I'd prefer it if we made a decision first. I'm not 100% sure if we want to change this and if yes to what.

[nikaltipar]

@nikaltipar I appreciate the PR but I'd prefer it if we made a decision first. I'm not 100% sure if we want to change this and if yes to what.

Understood, I was just looking for something unrelated and this request caught my eye for some reason.

[juansolm]

@nikaltipar good catch. However agg_list seems to only exist for Expr.map_batches.
And it's marked as deprecated so will probably be removed sometime in the future

I also found Expr.agg_groups, But this too will be deprecated in the future

This is Pandas: https://pandas.pydata.org/docs/reference/api/pandas.pivot_table.html
So your alternative: agg_function is also a good option