Fix syntax compatibility with rstan 2.26

[andrjohns]

This PR updates your Stan syntax for compatibility with the upcoming rstan 2.26, and removes files that are auto-generated by rstantools on installation.

Your model syntax was updated to to fix cases where a variable was declared with the same name as a function (e.g., log_h), which errors with the new parser

[Philip-Cooney]

@andrjohns Many Thanks for raising this issue and correcting the code. I am very much a newbie with Github. This current version on Github is actually behind the one on CRAN.

The reason for this is that I build the latest version of expertsurv a Fedora computer (which I submitted to CRAN) but can't upload this to Github because of the very large .so file (I replied in here but realize that I should have added an @ for you - https://discourse.mc-stan.org/t/so-file-size-when-building-an-r-package-that-uses-compiled-stan-code/16530/8).

If I could get the .so file version reduced, then I can upload the files to Github, and will be happy to make the changes you suggest.

[andrjohns]

No worries!

You don't need to be uploading/committing the compiled/objects code for R packages, since they will be built from source regardless. Additionally, the compiled object/.so file will only be compatible with the operating system that it was compiled on, so it's generally not too helpful.

For stan-based packages specifically, you also generally shouldn't be committing the src folder at all (unless you have additional custom code to be compiled) as the c++ for your model will be generated by rstantools when your package is installed.

[Philip-Cooney]

Fantastic! Thanks for the explanation. I'll fix this first this first thing Monday on the CRAN branch and then submit in a while when I have some other updates processed.