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