Feature/issue 861 erfc nan

#### Summary:
Fixes issue #861 . 

#### Intended Effect:
agrad/rev/erfc should now return NaN for gradients and value when passed in NaN.

#### How to Verify:
Run test/unit-agrad-rev/functions/erfc

#### Side Effects:
None.

#### Documentation:
None. Should document behavior of function when given NaN together with other functions.

#### Reviewer Suggestions:
Anyone