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
• _cdf functions using conditional | notation (i.e., _cdf(y |x))
  • The current CRAN rstan has a bug in recognising this in _cdf functions, so I've updated your syntax to work on the log scale with the _lcdf function. This will let your package be compatible with the current and future parser, and is also more numerically stable with extreme values (bonus!)

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!

[sempwn]

@andrjohns Great to hear that rstan 2.26 is now on CRAN. I've merged your PR and will release a new version to CRAN soon. Thank you for all the help!