Adds lazy version of aggregated_by routine to Cube.

[zklaus]

Closes #3280

Signed-off-by: Klaus Zimmermann [email protected]

[pp-mo]

I haven't had time to check code in depth yet,
but the idea is a really worthwhile + exciting ! 👍 🚀

Some tests for the new functionality will be wanted.

P.S. We just fixed the CI header tests, so sorry about that !

[zklaus]

Thanks! Yes, I am going through the checklist now and will add tests and whatsnew and whatnot :).

In fact, this is the only reason why this is marked as WIP, the code itself is considered finished and of course works for me.

Maybe to ease reading it: I completely kept the code for the non-lazy version, just put it in the else branch of an if statement guarding the lazy version in the exact same way as in the collapsed method.

A more aggressive version is possible were also the non-lazy part is implemented in the same way as the lazy part with only lazy_aggregate replaced by aggregate and da.stack replaced by np.stack.

[zklaus]

Ok, @bjlittle, @pp-mo, I think this is ready.

[lbdreyer]

Thanks for your work on this @zklaus ! This is a really nice, clean addition!

I just have a comment on the testing of 2D coordinates

[zklaus]

@pp-mo, @lbdreyer thanks for your help. I removed the spanning coordinates.

[pp-mo]

@zklaus great !
Thanks for this really neat improvement.