Implement $rolling() for DataFrame and LazyFrame

[etiennebacher]

No description provided.

[eitsupi]

Thanks for working on this.
I don't think this currently supports $ungroup(). Need to add tests.

Also, rather than making the implementation of RPolarsGroupBy complicated like this, I thought it would be simpler and better to delete by_agg on the Rust side and rewrite it to always call $lazy() and $collect() on the R side.

r-polars/src/rust/src/rdataframe/mod.rs

Lines 310 to 327 in ffc9a5c

	//used in GroupBy, not DataFrame
	pub fn by_agg(
	&mut self,
	group_exprs: Robj,
	agg_exprs: Robj,
	maintain_order: Robj,
	) -> RResult<RPolarsDataFrame> {
	let group_exprs: Vec<pl::Expr> = robj_to!(VecPLExprCol, group_exprs)?;
	let agg_exprs: Vec<pl::Expr> = robj_to!(VecPLExprColNamed, agg_exprs)?;
	let maintain_order = robj_to!(Option, bool, maintain_order)?.unwrap_or(false);
	let lazy_df = self.clone().0.lazy();
	let lgb = if maintain_order {
	lazy_df.group_by_stable(group_exprs)
	} else {
	lazy_df.group_by(group_exprs)
	};
	RPolarsLazyFrame(lgb.agg(agg_exprs)).collect()
	}

[etiennebacher]

I thought it would be simpler and better to delete by_agg on the Rust side and rewrite it to always call $lazy() and $collect() on the R side.

I'll open a new PR for that as it touches the $agg() behavior overall, not just for rolling computations. Would it be ok for you to have a "dirty" implementation of $ungroup() similar to the change in $agg() here in the meantime?

[eitsupi]

Would it be ok for you to have a "dirty" implementation of $ungroup() similar to the change in $agg() here in the meantime?

Of course it's okay. Thanks!