Could remove Rcpp usage entirely from ini_to_core_reader.cpp

[bpbond]

From #692 (comment)

Even simpler would be to rip out Rcpp usage entirely in this file, and just depend on std::filesystem! I couldn't quite get this to work — see #694 — but enough already, closing that PR but will open an issue for the future.

For the record, my code in #694 looked like this in ini_to_core_reader.cpp:

// Whether using the R package or building standalone Hector,
// use std::filesystem (which is available since the C++ 17 standard)
// if available; otherwise fall back to boost::filesystem (which
// needs to be installed).
#if __cpp_lib_filesystem || __has_include(<filesystem>)
#include <filesystem>
namespace fs = std::filesystem;
#else
#include <boost/filesystem.hpp>
namespace fs = boost::filesystem;
#endif

(and down below)

     // If the given path (absolute or relative) points to a file
      // that exists, use that. Otherwise, assume that the path
      // is relative to the INI file's directory.
      fs::path csvFilePath(csvFileName);
      if (!fs::exists(csvFilePath)) {
        fs::path iniFilePath(reader->iniFilePath);
        fs::path fullPath(iniFilePath.parent_path() / csvFilePath);
        csvFileName = fullPath.string();
      }

And in Makevars:

CXX_STD = CXX17