Feature/test nan fabs

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