Enable multi_gp in the language.
Add multi_gp to the manual.
Fix inverse in manual.
Fix function signature.
Add agrad unit tests for multi_gp_log.
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Fix neg_binomial_rng
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Unit test to check the NUTS slice criterion
fully specifying call to boost::math::pdf
Merge branch 'develop' of https://github.com/stan-dev/stan into develop