switched to no namespace version of atan2
added NaN tests for atan2
added NaN and derivative doc for atan2 in manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-atan2
Merge branch 'develop' into feature/test-nan-atan2
Merge pull request #966 from stan-dev/feature/test-nan-atan2

Feature/test nan atan2