added operator/ and operator\ for matrix matrix
added operator/ and operator\ for matrix matrix

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