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
added cdf_log test to distribution test_fixture
added cdf_log to normal distribution with test
vectorized exponential_cdf and added missing test
added cdf_log to exponential with test
added cdf_log with test for cauchy distribution
added cdf_log to beta distribution with test
current progress

Not entirely sure that the functions are producing the right results
added vectorized cdf to double exponential with test
added vectorized cdf_log to double exponential with test
vectorized cdf and use operands_and_partials for exp_mod_normal distr
added cdf_log to exp_mod_normal
added operands_and_partials to gumbel cdf
added cdf_log to gumbel distribution