added NaN test for multiply_log
added failing NaN test for agrad/rev/multiply_log
fixed agrad/rev/multiply_log so gradients return NaN if input is NaN
Merge branch 'feature/issue-845-multiply-log-nan' into feature/issue-847-fwd-functions-nan
updated ceil, floor, fmod, log_falling_factorial and fvar so fvar with value NaN sets deriv to NaN