implement adjoint ODE solver

[wds15]

Description

For more scalable ODE solvers in Stan, the adjoint ODE method as described in the design doc

https://github.com/stan-dev/design-docs/blob/master/designs/0027-adjoint-ode.md

should be implemented in stan-math.

Example

Please refer to the design doc for details.

Expected Output

Faster ODE results and their gradients for the case of many parameters and few ODE states.

Current Version:

v4.0.1

[wds15]

implemented with #1905