added new language features

- reject
- ode_solve
change ode_solve to integrate_ode
added snippets for integrate_ode and reject
fix integrate_ode number of parameters
updating to Stan model lang version 2.5.0 (but not functions)
updated functions to 2.5.0
updated to include new functions for 2.5.0
added append_(row|col) functions
added explanations of variables to integrate_ode snippet
keep distribution list sorted
bump version
Release the data in stan-lang.json under PDDL
Merge branch 'master' of github.com:stan-dev/stan-mode