Document/test unstable decimal dtype

[billylanchantin]

Discussed here: #1018 (comment)

Adds a test module and a warning to the moduledoc:

[billylanchantin]

This is actually a result of our expectations:

explorer/lib/explorer/backend/lazy_series.ex

Lines 772 to 783 in 020fb23

	defp dtype_for_agg_operation(op, _)
	when op in [:count, :nil_count, :size, :n_distinct, :argmin, :argmax],
	do: {:u, 32}
	defp dtype_for_agg_operation(op, series)
	when op in [:first, :last, :sum, :min, :max, :product],
	do: series.dtype
	defp dtype_for_agg_operation(op, _) when op in [:all?, :any?], do: :boolean
	defp dtype_for_agg_operation(:mode, series), do: {:list, series.dtype}
	defp dtype_for_agg_operation(_, _), do: {:f, 64}

I'm not entirely sure it's correct. But it seems to be?

>>> df = pl.DataFrame({"a": [1.1, 2.2]}, schema={"a": pl.datatypes.Decimal(18, 2)})
>>> df.with_columns(b=pl.col("a").mean())
shape: (2, 2)
┌───────────────┬──────┐
│ a             ┆ b    │
│ ---           ┆ ---  │
│ decimal[18,2] ┆ f64  │
╞═══════════════╪══════╡
│ 1.10          ┆ 1.65 │
│ 2.20          ┆ 1.65 │
└───────────────┴──────┘

[josevalim]

It is fine to default our decimal operations to float and fix them as Polars start returning decimals for them too. I would prefer that than raising (because returning float still "works").

[billylanchantin]

@josevalim Sorry forgot about this. Are you saying we should remove this test and force our decimal operations to return float instead?

[josevalim]

I think we should return whatever Polars return for now and we will fix it naturally as Polars fixes it.

[billylanchantin]

Ok, then the test can stay. When Polars fixes their end, we will learn about it because the test will break. Then we can convert it to a test of what the behavior should actually be.