Feature/test nan asin

#### Summary:
Added NaN tests for asin in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. I also added more boundary tests to verify derivative behavior outside of [-1,1]. I also switched to the no namespace version of asin to match the double version that is called.

#### Intended Effect:
Added NaN tests for asin in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. 

#### How to Verify:
Run test/unit-agrad-fwd/functions/asin, test/unit-agrad-rev/functions/asin

#### Side Effects:
None.

#### Documentation:
Added doc on behavior of asin at NaN and partial derivatives.

#### Reviewer Suggestions:
Anyone.