Merge branch 'develop' into feature/test-nan-op-lte
added NaN test for logical_lte
add NaN test for operator_lte
added NaN test for operator_lte
added doc for operator_lte
Merge branch 'develop' into feature/test-nan-op-lte
Merge pull request #1009 from stan-dev/feature/test-nan-op-lte

Feature/test nan op lte