added extra vectorized signatures to sum, prod, min, max, sd, variance, mean and cleaned up fourth arg formatting on functions
added extra vectorized signatures to sum, prod, min, max, sd, variance, mean and cleaned up fourth arg formatting on functions

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