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 failing NaN test for agrad/rev/fmax
fixed agrad/rev/fmax so gradients return NaN when input is NaN
Merge branch 'feature/issue-845-fmax-nan' into feature/issue-847-fmax-nan
added failing nan test for agrad/fwd/fmax
fixed agrad/fwd/fmax so derivatives return nan for nan input