Merge branch 'develop' into feature/test-nan-op-ne
added NaN test for logical_neq
add NaN test for operator_not_equal
added NaN test for operator_neq
added doc for operator_neq
Merge branch 'develop' into feature/test-nan-op-ne
Merge pull request #1012 from stan-dev/feature/test-nan-op-ne

Feature/test nan op ne