using enable_if version of agrad::var ctor for full generality across platforms
using enable_if version of agrad::var ctor for full generality across platforms

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