functionality for the derivative of atan2

[huckl3b3rry87]

I am a JuMP user that wanted to be able to use the atan2 function and I noticed that it was on someones radar here to add this functionality and I am wondering is this straightforward to do or would it be difficult to do and extend to JuMP since atan2 is a function of two variables. @mlubin what do you think?

[mlubin]

JuMP doesn't take derivatives for 2-argument functions from this package, so adding it here won't get you too much. The whole of JuMP's NLP code is slated to be replaced by @jrevels (this will be the 4th rewrite of JuMP's AD), so I won't be investing much in adding functionality to the existing code. You should be able to get atan2 working via user-defined functions.