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
updated fwd/matrix tests
fixed value_of NaN test
Merge branch 'develop' into feature/test-nan-abs
Merge branch 'develop' into feature/test-nan-abs
Merge branch 'develop' into feature/test-nan-abs
fixed eigenvalues_sym, eigenvectors_sym, and singular_values test
Merge branch 'develop' into feature/test-nan-abs
updated NaN tests
Merge pull request #946 from stan-dev/feature/test-nan-abs

Feature/test nan abs