Bugfix/issue 882 trunc nan

#### Summary:
Fixes issue #882 . 

#### Intended Effect:
agrad/rev/trunc should now return NaN when passed in NaN. The same goes for agrad/fwd/trunc.

#### How to Verify:
Run test/unit-agrad-rev/functions/trunc and test/unit-agrad-fwd/functions/trunc

#### Side Effects:
None.

#### Documentation:
None. Should document behavior of function when given NaN together with other functions.

#### Reviewer Suggestions:
Anyone