Document C++ functions

[fboehm]

Maybe I can use "tags" @author Fred boehm, etc. and then get Roxygen2 to document things?? I need to try this!

[fboehm]

@PARAM X ..., etc.

[fboehm]

https://cran.r-project.org/web/packages/roxygen2/vignettes/roxygen2.html

This tells you how to use Roxygen2 with Rcpp