Fix bug where loop counter is incremented twice per iteration (#816)

Between full tests, doing fine on this ODE model, and the survival model I ran that didn't work prior to the fix I feel good about the checks here.

Clean up lint warnings from CLion (clang-tidy, etc.)

Feature/issue 778 add mpi build, addresses issue #778 

See #816 for more details and discussion.

Mark relevant numeric_limits methods constexpr

This works now that stan is C++11. Previously, there was no
way to do the equivalent with doubles since we could do static
compile-time methods like this for integral and enum types.

Fix linking issue where static members are defined but not initialized

In particular, some members were declared but not defined, which
sometimes causes a linker error. See #806 for more discussion.

changes to opencl_context_rewrite

Revert the matrix autos from #801.

license link in README, fixes #802

Change so that there is a singleton base class and then an class that acts as an API to that base class