Merge branch 'develop' into feature/test-nan-fmod
added NaN test for fmod
added failing NaN test for agrad/rev/fmod
fixed agrad/rev/fmod so gradients return NaN for NaN inputs
fixed fmod so gradients return NaN for input NaN
cherry-pick agrad/fwd/functions/value_of from GeneralizeOperandsAndpartials Branch
added NaN test for fmod
added doc for fmod
Merge branch 'develop' into feature/test-nan-fmod
updating includes for fmod
Merge branch 'develop' into feature/test-nan-fmod
Merge pull request #1001 from stan-dev/feature/test-nan-fmod

Feature/test nan fmod