added NaN test for fma
added NaN test for fma
added NaN test for fma
added failing NaN test for agrad/rev/fma
fixed typo in test/unit-agrad-rev/nan_util
fixed agrad/rev/fma so gradients return nan
added NaN and derivative doc for fma to manual
Merge branch 'develop' into feature/test-nan-fma
Merge branch 'develop' into feature/test-nan-fma
Merge pull request #973 from stan-dev/feature/test-nan-fma

Feature/test nan fma