Feature/test nan atan2

#### Summary:
Added NaN tests for atan2 in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. I also switched to the no namespace version of atan2 to match the double version.

#### Intended Effect:
Added NaN tests for atan2 in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. 

#### How to Verify:
Run test/unit-agrad-fwd/functions/atan2, test/unit-agrad-rev/functions/atan2

#### Side Effects:
None.

#### Documentation:
Added doc on behavior of atan2 at NaN and partial derivatives.

#### Reviewer Suggestions:
Anyone.