Rewrite dynamic programming routine in C++

[huisaddison]

Ryan’s C implementation is included as-is. In order for C++ code in other packages to use the C implementation, it must be exported by Rcpp using the // [[Rcpp::export]] attribute. This directs Rcpp to build the necessary header files, which are made available to other R packages via “LinkingTo”.

However Rcpp doesn't play well with points as function parameters (it needs to map each parameter to its R equivalent, and doesn't know what to do with pointers). This doesn’t work even if we ask only for a C++ interface (i.e., using the Rcpp::interfaces attribute).

Solution: Rewrite tf_dp.c to use modern C++ structures (i.e., vectors and reference semantics), and make that available through an Rcpp header. This is the preferred solution (and can be implemented without changing the R frontend).

Alternative solution: Leave the implementation as-is and distribute a copy of the C code with each R package that requires it (e.g., trendfilter).