macOS RcppArmadillo installation + using compiled code

[coatless]

If you are encountering issues installing RcppArmadillo on macOS, please use the macrtools package here to install it:

https://mac.thecoatlessprofessor.com/macrtools/

You can read more about setting up compiled code on mac here:

https://thecoatlessprofessor.com/programming/r-compiler-tools-for-rcpp-on-macos/