Merge branch 'develop' into feature/test-nan-op-eq
added NaN test for logical_eq
add NaN test for operator_equal
added NaN test for operator_eq
added doc for oeprator_eq
Merge branch 'develop' into feature/test-nan-op-eq
Merge pull request #1011 from stan-dev/feature/test-nan-op-eq

Feature/test nan op eq