Fully delegate installation rstantools

[andrjohns]

This PR updates your package to fully delegate the installation/compilation of stan models to rstantools.

Note that your package appears to be missing a utility_FA.cpp file and does not build

[bgoodri]

We need this PR to be merged and released to CRAN in order to release the StanHeaders that has been years in the making but is still years behind the current Stan development branch. Please do so this week.

[andrjohns]

FYI that your latest CRAN release removed rstantools from your DESCRIPTION file's Imports, which means that your package will fail to compile unless the user already has rstantools installed - it won't be installed as part of your package's dependencies. We strongly recommend that you re-add rstantools to your Imports

[andrjohns]

Additionally, your package is still failing to build against the next StanHeaders because there are compilation flags missing from your Makevars. We recommend using the rstantools-based approach from this PR so that your package always remains compatible