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 #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
Merge branch 'develop' into feature/issue-847-fwd-functions-nan
added NaN tests for round and trunc
added NaN test for fdim