Feature/test nan cos

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