new refine entry for the atan2 function

- This resolves #8272
- This is linked with the previous PR #8273
but maybe you only want to do that if the ratio has a leading negative:

```
from sympy.core.function import _coeff_isneg
...
rat = args[0]/args[1]
if _coeff_isneg(rat):
    return -atan(-rat) + S.Pi
return atan(rat) + S.Pi
```