added failing NaN test for agrad/rev/fmax
fixed agrad/rev/fmax so gradients return NaN when input is NaN
added failing NaN test for fmin
fixed agrad/rev/fmin so gradients return NaN when input is NaN
add similar NaN testing framework
expanded NaN testing framework to handle 3 variables
updated nan testing framework to check gradients of nan functions up to third order
updated nan testing framework for fvar<fvar<double> > and fvar<fvar<var> >
updated fvar so if value is nan, then deriv is set to nan too
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
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
fixed merge conflict for fmin
fixed merge conflict for fmax