Mean

[LukeMathWalker]

Upstream mean from ndarray-stats to ndarray.
Breaking change on mean_axis: it returns Option<Array<A, D:Smaller>> now.

[LukeMathWalker]

The CI run on nightly seems to be failing for unrelated reasons (failing to install Rust itself).

[jturner314]

The CI issue (due to rust-lang/rust#57488) is now fixed, so I re-ran CI.

I added a few comments. Additionally, if we add tests/numeric.rs (which is a good idea IMO), we should also move the relevant tests in tests/array.rs to it (e.g. sum_mean, sum_mean_empty, var_axis, std_axis, etc.).

Everything else looks good to me.

[LukeMathWalker]

All done - I have moved all the numeric tests I could spot in array.rs to numeric.rs.

[jturner314]

Looks good to me. Once this is merged, I think we should also change var_axis and std_axis to return an Option so that they're consistent with mean_axis. [Edit: They do require A: Float, though, so this isn't strictly necessary.]

[LukeMathWalker]

I am happy to do the changes to var_axis and std_axis in this PR @jturner314, if this makes the merging of PRs with breaking changes easier (less coordination required).

[jturner314]

I'm not quite sure why I waited to merge this. (Maybe I was waiting on #577?) Anyway, let's go ahead and merge it.

Thanks for your work on this @LukeMathWalker!

Edit: I've rearranged the commit history to keep some commits separate but combine several small ones.