Deprecate atan2

[benchislett]

To be consistent with Julia's atan(x, y) function, I renamed atan2 to atan.
Here's the help message for atan:

atan(y)
atan(y, x)

Compute the inverse tangent of y or y/x, respectively.

For one argument, this is the angle in radians between the positive x-axis and the point (1, y), returning a value in the interval [-\pi/2, \pi/2].

For two arguments, this is the angle in radians between the positive x-axis and the point (x, y), returning a value in the interval [-\pi, \pi]. This corresponds to a standard
atan2 (https://en.wikipedia.org/wiki/Atan2) function.

[maleadt]

Hmm, I wonder if we should try and have some deprecation mechanism for changes like this.

[Sleort]

What's the status here? It would be nice to have this PR merged...

(Regarding deprecation, can't we just use @deprecate atan2(y, x) atan(y, x)?)

[maleadt]

No, @depwarn doesn't work on the GPU. We should probably have both definitions until there's a breaking release.