Added release notes for 2.24.0

RELEASE-NOTES.txt

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+STANC3 RELEASE NOTES
+======================================================================
+
+v.2.24.0 (28 July 2020)
+======================================================================
+
+New functions: 
+   `algebra_solver_newton`
+
+   `matrix hmm_hidden_state_prob(matrix, matrix, vector)`
+   `int[] hmm_latent_rng(matrix, matrix, vector)`
+   `real hmm_marginal(matrix, matrix, vector)`
+
+   `T lambert_w0(T)`, for any T
+   `T lambert_w1(T)`, for any T
+
+   `matrix matrix_power(matrix, int)`
+
+   `real multinomial_logit_log(int[], vector)`
+   `real multinomial_logit_lpmf(int[], vector)`
+   `int[] multinomial_logit_rng(vector, int)`
+
+   `ode_adams`, `ode_adams_tol`
+   `ode_bdf`, `ode_bdf_tol`
+   `ode_rk45`, `ode_rk45_tol`
+
+   `T pow(int, T)`
+   `T pow(real, T)`
+   `T pow(T, int)`
+   `T pow(T, real) `
+    (previously only real pow(real, real) was available)
+
+   `T reverse(T)`, where T is vector, row_vector or an array of any type
+
+   `matrix identity_matrix(int)`
+   `real[] linspaced_array(int, real, real); row_vector linspaced_row_vector(int, real, real); vector linspaced_vector(int, real, real)`
+   `int[] one_hot_int_array(int, int); real[] one_hot_array(int, int); row_vector one_hot_row_vector(int, int); vector one_hot_vector(int, int)`
+   `int[] ones_int_array(int); real[] ones_array(int); row_vector ones_row_vector(int); vector ones_vector(int)`
+   `vector uniform_simplex(int)`
+   `int[] zeros_int_array(int); real[] zeros_array(int); row_vector zeros_row_vector(int); vector zeros_vector(int)`
+
+
+New features:
+
+- pedantic mode
+- experimental optimization mode
+- new integer division operator `%/%`
+- non-scalar values are now allowed in lower, upper, offset and multiplier
+- integrate_ode_* functions are now deprecated
+- data-only restricted functions now work with parameters in generated quantities
+- improved out-of-bounds messages for vectors, matrices
+- added checks for invalid literals
+- `--allow_undefined` and `--include_paths` are deprecated in favor of `--allow-undefined` and `--include-paths`
+- cleaned up the generated model C++ code: model is now final, using statements instead of typedef, better use of inline, use of `.clear()` instead of `.resize(0)`, remove unnecessary functions
+- cleaned up the OCaml code: better use of helper functions, removed duplicate functions, better exception messages, replace imperative code for handling Stan Math signatures
+- size expressions for parameters and generated quantities are now only evaluated once
+- better handle model names that are not C++ friendly (start with numbers, have non-alphanumeric characters)
+
+Bugfixes:
+- changed the loop index type to int to prevent issues when using it in expressions with signed numbers
+- integer arrays now initialize to the smallest representable integer
+- var arrays initialize with a prebuilt dummy var, to not creat new varis
+- all map_rect calls are registered when using MPI
+- _lpdf/_lpmf functions are now used instead of *_log when generating code for the tilde statements
+- single argument distribution do not require the vertical bar anymore (e.g. std_normal_lpdf(x|))
+- fixed a bug that caused tilde statements to conflict with userdef functions
+- rng functions are no longer allowed in size declarations
+- fixed the _lpdf functor struct so that UDF with _log and int argument now compile
+- fixed array literal type-checking 
+- fixed the error messages for missing input (line number is now displayed properly)
+- fixed indexing in transform_init()
+- fixed emitting generated quantities only (for gq_writer)
+- fixed debug data generator for constrained matrix types and added array value bounds