added NaN test for abs
update nan test for agrad/rev/abs
added NaN test for agrad/fwd/abs
cherry-pick agrad/fwd/functions/value_of from GeneralizeOperandsAndpartials Branch
added tests for value_of
cherry-pick 50d0cb02c53f7b8a21264914cf2a3750eff394e5
added NaN and derivative doc for abs
switched to no namespace version of asinh
added NaN test for asinh
added NaN and derivative doc for asinh in manual
switched to no namespace version of atan
added NaN tests for atan
added NaN and derivative doc for atan to manual
switched to no namespace version of atanh
added NaN tests for atanh and updated bound tests
added NaN and derivative doc for atanh in manual
added NaN test for digamma
added NaN test for digamma
added NaN and derivative doc for digamma in manual
cherry-pick agrad/fwd/functions/value_of from GeneralizeOperandsAndpartials Branch