adding failing fdim test
fixed implementation of fdim by calling the version in math.h
adding a check for isnan because Windows operates differently than Mac
fixed fdim test
added failing test
fix agrad/rev round
added failing test
switched to no namespace version of trunc
updated agrad/fwd/round
added NaN test for round in agrad/fwd
added NaN test for trunc in agrad/fwd
add failing NaN tests for rev/fdim
fixed rev/fdim to return NaN for any NaN inputs
Merge pull request #870 from stan-dev/feature/issue-853-fdim-nan

Fixes #853. Feature/issue 853 fdim nan
Merge pull request #883 from stan-dev/bugfix/issue-881-round-nan

Fixes #881. Bugfix/issue 881 round nan
Merge pull request #884 from stan-dev/bugfix/issue-882-tru-nan

Fixes #882. Bugfix/issue 882 trunc nan
Merge pull request #888 from stan-dev/bugfix/issue-887-fdim-rev-nan

Fixes #887. Bugfix/issue 887 fdim rev nan
fix merge conflict