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. 
Suppose x and y are positive.  Then your code for `atan2(y, -x)` gives `tan(-y/x) + pi`.  You may want turn this into `-tan(y/x) + pi`, isn't?