added NaN test for Phi
added NaN testing framework for agrad/rev/functions
updated NaN testing framework for Phi
added nan doc to manual for derivative of Phi
fixed typo in doc for Phi derivative
added NaN test for ceil
add failing NaN test for ceil
fixed NaN gradient for ceil using unlikely()
Merge branch 'feature/issue-845-ceil-nan' into feature/issue-847-fwd-functions-nan
fixed ceil so gradients return NaN