Added vectorization to multi_normal_log
Vectorization of multi_normal, multi_normal_prec and multi_normal_cholesky + tests
vectorization of multi_student_t_log
Changed "response variable" to "random variable" to be consistent with the Stan
added doc for vectorized multi_normal and added subsection in vectorization chapter to describe how it works
added doc for vectorized multi_normal_prec, multi_normal_cholesky, multi_student_t
Fix some unused variable warnings
Some small improvements to multi_normal and other vectorized multivate
Some other small improvements to multi_normal and other vectorized multivate
Added more tests to basic_estimators/normal_multi.stan
Add gradient tests to vectorized multivariate distributions (multi_normal and friends)
Fix issue 305: remove unused multi_normal over matrix outcomes
Some more unit tests for multivariate distributions
Merge branch 'develop' into feature/fix-issue-589-vectorize-multi_normal

Conflicts:
	src/stan/gm/function_signatures.h
	src/test/unit/prob/distributions/multivariate/continuous/multi_normal_cholesky_test.cpp
multi_normal_cholesky: this test was added again by mistake when soving merging conflicts
Added mixed double/var tests for multi_normal
add mixed type unit tests for multi_normal
add mixed type unit tests for multi_normal_cholesky and multi_normal_prec
add mixed type unit tests for multi_student_t
Merge branch 'develop' into feature/fix-issue-589-vectorize-multi_normal

Conflicts:
	src/stan/gm/function_signatures.h