Update deprecated syntax for future rstan compatibility

[andrjohns]

Now that rstan 2.26 is available on CRAN we need to update the deprecated syntax in your package's Stan models, otherwise it will fail to install with an upcoming version of RStan.

The following updates have been made:

• New array syntax
• integrate_ode_rk45 replaced by ode_rk45

Note that the new ode_rk45 allows for the inputs to be passed directly (instead of packing them into the theta, x_i, x_r structures), but I've only updated your syntax to work with the current structures. For more guidance on using the new ODE interfaces you can see this case study: https://mc-stan.org/users/documentation/case-studies/convert_odes.html

More information about the deprecated and removed syntax in Stan can be found here:

• https://mc-stan.org/docs/functions-reference/deprecated-functions.html
• https://mc-stan.org/docs/functions-reference/removed-functions.html

If you could merge and submit to CRAN soon, it would be greatly appreciated.

Let me know if you have any questions about these changes.

Thanks!