Add squared_dist.hpp file.
Add squared_dist.hpp file.

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