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
added NaN test for digamma
added NaN test for digamma
added NaN and derivative doc for digamma in manual
switched to no namespace version of atan2
added NaN tests for atan2
added NaN and derivative doc for atan2 in manual
added NaN test for falling_factorial
added NaN test for falling_factorial
added NaN and derivative doc for falling_factorial in manual
updated fwd/matrix tests
updated fwd/matrix tests
updated fwd/matrix tests
updated fwd/matrix tests