add NaN test for operator_greater_than
added NaN test for logical_gt
added NaN test for operator_gt
added doc for operator_gt
Merge branch 'develop' into feature/test-nan-op-gt
Merge branch 'develop' into feature/test-nan-op-gt
Merge pull request #1008 from stan-dev/feature/test-nan-op-gt

Feature/test nan op gt