This reverts commit 52a3b862c401bf73f92e63b57487b3ac179d72a4.

Conflicts:
	src/docs/stan-reference/distributions.tex
	src/stan/gm/function_signatures.hpp
	src/stan/prob/distributions/multivariate/continuous/gaussian_dlm_obs.hpp
	src/test/prob/distributions/multivariate/continuous/gaussian_dlm_obs_test.cpp