add higher order autodiff tests for append_row and append_col
added higher order autodiff tests for: accumulator, array_builder, assign, diag_post_multiply
trying to fix LDLT_factor so it works with higher order autodial
added more higher order autodiff matrix tests
added quad_form_diag higher order autodiff tests
added qr_Q and qr_R higher order autodiff tests
added higher order autodiff tests for: mdivide_left_spd, mdivide_right_spd, mdivide_right_tri, mdivide_left_tri
added higher order autodiff tests for log_determinant_spd
added higher order autodiff tests for log_determinant_ldlt and fill
added higher order autodiff tests for get_base1_lhs, get_base1, initialize
added higher order autodiff tests for mdivide_left_ldlt and mdivide_right_ldlt
added higher order autodiff tests for trace_inv_quad_form_ldlt and trace_gen_inv_quad_form_ldlt
clean up some matrix files
Merge branch 'develop' into feature/issue-824-check-matrix-autodiff
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