rename isnull and isnan methods

[MarcoGorelli]

as discussed on the last call

[rgommers]

I don't remember what the rationale for this was, can you add one @MarcoGorelli? These methods are using the no-underscore flavor pretty much universally I think, and also in the array API for isnan.

[MarcoGorelli]

just snake-case - you'd read it as "is null", and replacing the space with an underscore, would get "is_null"

[rgommers]

Hmm - I'd think that that's probably not a good reason for is and has, they're kinda special in Python (isinstance, issubclass, hasattr, etc.) - that plus isnan/isnull being the current names everywhere means we should probably leave this as is.

[MarcoGorelli]

OK sure, closing then

[jorisvandenbossche]

everywhere

Just to be pedantic, but not "everywhere": for example pyarrow and polars both use "is_null" etc (but yeah, those are certainly the outliers and probably less relevant for the exercise here)