added NaN test for Phi_approx
removing unused function in Phi_approx_test.cpp
added NaN doc for Phi_approx
added NaN testing framework for agrad/rev/functions
added NaN test for Phi_approx
updated NaN test for agrad/rev/Phi_approx
Merge branch 'feature/issue-847-fvar-nan' into feature/issue-843-test-nan-Phi-approx
added NaN testing framework for fwd mode autodiff
added NaN test for agrad/fwd/Phi_approx
fixed minor typo in appendix for Phi_approx
updated fwd/matrix tests
Merge branch 'develop' into feature/issue-843-test-nan-Phi-approx
Merge pull request #944 from stan-dev/feature/issue-843-test-nan-Phi-approx

Feature/test nan phi approx