Variadic argument lists for ODEs

[bob-carpenter]

Description

Provide a way to use variadic argument lists for odes to avoid the inconvenient packing/unpacking that goes along with using the current ODE solvers.

Moved here from PR #1641.

Example

real[ , ] integrate_ode_rk45(function ode, real[] initial_state,
                             real initial_time, real[] times,
                             T1 arg1, T2 arg2, ...)

where the ode right hand side function looks like:

real[] ode(real time, real[] state, T1 arg1, T2 arg2, ...)

We'll need to create new signatures for these functions to handle the issue with inferring types described https://discourse.mc-stan.org/t/default-arguments-parameter-packs/12748/14.

Current Version:

v3.1.0

[bbbales2]

Closed by #1641