Expose ddof on covariance

[cigrainger]

Completes #743

I've got a couple failing tests but won't be able to get to it right away.

[philss]

I think one of the failures it's because we are transforming every argument in an expression. Maybe we need to do just like we did with correlation here:

explorer/lib/explorer/polars_backend/expression.ex

Line 206 in 78d9591

def to_expr(%LazySeries{op: :correlation, args: [series1, series2, ddof]}) do

, and have a custom implementation.

The other test is just the arity, for what I could understand.

[cigrainger]

I think one of the failures it's because we are transforming every argument in an expression. Maybe we need to do just like we did with correlation here:

explorer/lib/explorer/polars_backend/expression.ex

Line 206 in 78d9591

def to_expr(%LazySeries{op: :correlation, args: [series1, series2, ddof]}) do

, and have a custom implementation.
The other test is just the arity, for what I could understand.

Yes! Just realised that then saw your comment. Would have saved me a little bit 😂. Anyway should pass now. Thanks!