Merge branch 'develop' into feature/test-nan-hypot
switched to no namespace version of hypot
cherry-pick  bc17118d99d89bd51b576661ddb8c697b4de0cdd
added NaN test for hypot
added doc for hypot
Merge branch 'develop' into feature/test-nan-hypot
Merge pull request #1005 from stan-dev/feature/test-nan-hypot

Feature/test nan hypot