Feature/test nan op lt

#### Summary:
Added NaN tests for operator_less_than in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN.

#### Intended Effect:
Added NaN tests for operator_less_than in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. 

#### How to Verify:
Run test/unit-agrad-fwd/functions/operator_less_than, test/unit-agrad-rev/functions/ibeta

#### Side Effects:
None.

#### Documentation:
Added doc on behavior of operator_less_than at NaN.

#### Reviewer Suggestions:
Anyone.