fixed NaN bug in agrad/rev/acosh and switched to no namespace version of acosh
fixed NaN bug in agrad/rev/acosh and switched to no namespace version of acosh

stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math