New refine entry for the atan2 function

This PR adds the function refine_atan2. Compared with  PR #8476 in this one the functions is more clearly written with consisten variable names y, x. The doc string function is also corrected. 
Oh yes, this is due to automatic evaluation.  Perhaps, @smichr assumed you should use `evaluate=False` kwarg for atan.  But we don't do this for other refine helpers...  I don't know :(

Again, I'm already +1 on your changes as is.