Merge branch 'develop' into feature/issue-823-check-multi-prob-autodiff
added basic higher order autodiff tests for multi_normal
added basic higher order autodiff tests for inv_wishart, dirichlet, gaussian_dlm_obs, inv_wishart, lkj_corr, matrix_normal
fixed some agrad/fwd/matrix functions so they work with multivariate prob functions
added basic higher order autodiff tests for multi_gp, multi_gp_cholesky
added basic higher order autodiff tests for rest of multivariate distributions
Merge branch 'develop' into feature/issue-823-check-multi-prob-autodiff
Merge branch 'develop' into feature/issue-823-check-multi-prob-autodiff
Merge pull request #855 from stan-dev/feature/issue-823-check-multi-prob-autodiff

Fixes #823. Feature/issue 823 check multi prob autodiff