new refine entry for the atan2 function

- This resolves #8272
- This is linked with the previous PR #8273
On Wed, Nov 19, 2014 at 05:30:16PM -0800, Christopher Smith wrote:
>    To avoid confusion, you should catch the args as y, x = expr.args and then
>    refer to them that way in the code that follows.

I think it's fine.  args[0] is much more clear (vs y) - it's the first
argument.  I don't see any confusion.