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. 
This is the output for the four posible cases: 
```python
In [9]: refine(atan2(-y,x), Q.positive(y) & Q.positive(x))
Out[9]: -atan(y/x)

In [10]: refine(atan2(y,-x), Q.positive(y) & Q.positive(x))
Out[10]: -atan(y/x) + pi

In [11]: refine(atan2(-y,-x), Q.positive(y) & Q.positive(x))
Out[11]: atan(y/x) - pi

In [12]: refine(atan2(y,x), Q.positive(y) & Q.positive(x))
Out[12]: atan(y/x)
```