adding failing fdim test
fixed implementation of fdim by calling the version in math.h
added failing test
switched to no namespace erf in math.h
updating arithmetic in owens_t
updating arithmetic in owens_t
adding a check for isnan because Windows operates differently than Mac
updating includes
fixed fdim test
Merge pull request #873 from stan-dev/feature/issue-860-erf-nan

Fixes #860. Feature/issue 860 erf nan
Merge pull request #870 from stan-dev/feature/issue-853-fdim-nan

Fixes #853. Feature/issue 853 fdim nan
Merge branch 'develop' into feature/issue-405-refactor-init