Add interpolation methods

[CangyuanLi]

Reach feature parity with https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.interpolate.html

Working on this on the interpolation branch!

Since pandas uses the index as the x values, I think we should follow pola-rs/polars#16313.

[abstractqqq]

How is this going? I finished the kdtree refactor and have some time on other stuff

[CangyuanLi]

hmm, I have written all the code at least, but haven't verified it is correct yet--not sure how much re-writing would get it to match scipy exactly. I haven't had the time to work on it for a while :(. I can commit what I have so far if you also want to work on it

[abstractqqq]

No worries. I will do a review once you are ready to merge.

[abstractqqq]

The branch is here.

Will work on it once I get more time.

[abstractqqq]

@CangyuanLi

Just want to make sure, the use case is that we fit the spline on the data points, which may have nulls. Then we fill the nulls using the spline that we fit. Correct?

[CangyuanLi]

Yes, exactly!