atan2(0, 0) should return D('NaN') in some contexts

```
atan2(0, 0) currently returns 0, the same result given by the standard 
math module. Presumably this is to make the function continuous.

The actual indefinite value D('NaN') should be returned in the appropriate 
contexts.
```

Original issue reported on code.google.com by `exogen@gmail.com` on 30 Sep 2006 at 8:30
