C++ linkable package (interfaces)

[gvegayon]

The idea is that the core functions that are written in C++ should be available to be used in other projects based on Rcpp. The way this is done is detailed in section 3.6 Providing a C++ Interface https://cran.r-project.org/web/packages/Rcpp/vignettes/Rcpp-attributes.pdf of the Rcpp vignette Rcpp-attributes.