new refine entry for the atan2 function

- This resolves #8272
- This is linked with the previous PR #8273
Since we are working with a symbolic expression at this point there is nothing to be gianed by making the expression canonical wrt sign but there is a representational simplicity that can be achieved if we just check if there is a physical negative sign showing. I never made that private function public because I didn't know if it would be of greater beyond internal workings.