initial work
model compiles
Merge remote-tracking branch 'remotes/stan-dev/develop' into feature/dlm

Conflicts:
	src/stan/prob/distributions/multivariate/continuous.hpp
misc
working implementation
getting sequential version to work
update signatures
current progress

Not entirely sure that the functions are producing the right results
misc
rename gaussian_dlm_seq_log back to gaussian_dlm_log
fix ref to gaussian_dlm_seq_log
unit tests
replace inversion of Q with cholesky decomposition and solving
Merge remote-tracking branch 'remotes/stan-dev/develop' into feature/dlm

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update
added example models
updated DLM test for sizes of matrices
updating tests
added docs for gaussian_log_glm
clean up code