Add atan support for missings

[nicholasbalasus]

allowing atan to deal with two missing arguments

[oscardssmith]

This seems reasonable given that we already define atan(missing).

[garrison]

Thank you for the pull request (the first ever submitted from your account, according to GitHub!).

• Is there any reason not to have it return missing as well if only one of the provided arguments is missing?
• Might it be worth adding test(s)?

[garrison]

This line would be more consistent with the rest of the julia code base if a space were added after the comma. (Although there are a handful of exceptions, the space is present much more often than it is not.)

[nicholasbalasus]

Thank you, garrison! I have just added support for the cases where one of the values is missing and the other is not. I believe ::Number is better for this than ::Any but am not sure!

atan(::Missing, ::Missing) = missing
atan(::Number, ::Missing) = missing
atan(::Missing, ::Number) = missing

[garrison]

I agree that ::Number makes the most sense, as it makes this consistent with other two-argument math functions (e.g. *, /).

[garrison]

I think the easiest (and, in my opinion, the most appropriate) place to add a test case would be to add it to the list of "arithmetic operators". If this path is chosen, I think the phrase "arithmetic operators" should perhaps be renamed to "binary operations" (i.e., within the variable name, testset name, and comments). Note that the single argument form of atan is tested later in the same file, under "elementary functions".

EDIT: Actually, I believe it makes better sense to start a testset for "two-argument functions" right below "elementary functions." Others for which missing support could be implemented in the future include log and hypot. I'll leave the links to the "arithmetic operator" tests above because that's a good example of what to emulate.

[nicholasbalasus]

Added now (and it looks like the checks passed now after I had quite a few issues with the white space bot, my apologies!)

as it’s written now, I left atan in the “elementary functions” test set as well for the one input test, though this could definitely be changed if it would be better practice to only have it in one place.