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
added NaN tests for agrad/fwd/acosh
fixed NaN bug in agrad/rev/acosh and switched to no namespace version of acosh
updated bound tests for acosh
added NaN and derivative doc for acosh in manual
added NaN test for asin
added NaN test to agrad/fwd/asin
updated NaN test for agrad/rev/asin
switched to no namespace version of asin
updated bound tests for asin
added NaN and derivative doc for asin in manual
switched to no namespace version of asinh