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 acos
update nan test for agrad/rev/acos
added NaN test for agrad/fwd/acos
switched to no namespace version of acos to match double call
added bound tests for acos
added NaN doc and derivative doc for acos
final tests for nested reverse mode in place and passing
Merge branch 'feature/issue-771-ode-solver' of https://github.com/stan-dev/stan into feature/issue-771-ode-solver
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-771-ode-solver
fixed minor typo in appendix for Phi_approx
added NaN test for acosh
updated nan test for acosh
updated agrad/fwd/acosh