agg is too complex for people and llms

[dmpetrov]

Description

It's hard to comprehend that agg() should return iterator/list, not a single value per class. We need to change that.

I'd propose deprecating agg() via warning for now, introduce a new, value-returning function. And then rename the function to agg() and the current agg() to something like agg_n() or just remove it.

The value-returning aggregate function name could be acc() (accumulate), fus (fuse), dst (distil), red() (reduce), agg1() (silly), grp() (conflicts with group_by).