added NaN test for Phi
updated NaN testing framework for Phi
added NaN test for agrad/fwd/Phi
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-Phi-agrad
Merge branch 'develop' into feature/test-nan-Phi-agrad
Merge pull request #945 from stan-dev/feature/test-nan-Phi-agrad

Feature/test nan phi agrad
Merge branch 'develop' into feature/test-nan-abs