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
• offset is a reserved keyword, changed to input_offset

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!

[ConnorDonegan]

Thanks for this! I'm very grateful for this support.

Is Sept. 24 soon enough to avoid any trouble for you? If so, I'll update it by then (along with some other minor changes); otherwise I'll just update the package right away.

[andrjohns]

No worries! You don't need to worry about updating the package immediately, we still have one more major rstan release before this is an issue