message: add datetime to CSV
diff: #include <stan/services/sample/hmc_static_unit_e.hpp>
  #include <stan/services/sample/hmc_static_unit_e_adapt.hpp>
  #include <stan/services/sample/standalone_gqs.hpp>
  +#include <chrono>
  #include <fstream>
  #include <memory>
  #include <sstream>
  @@ -229,6 +230,13 @@ int command(int argc, const char *argv[]) {
  write_stan(sample_writer);
  write_model(sample_writer, model.model_name());
  +
  + // write current datetime to the CSV
  + const std::time_t current_datetime = std::chrono::system_clock::to_time_t(std::chrono::system_clock::now());
  + std::stringstream current_datetime_msg;
  + current_datetime_msg << std::put_time(std::localtime(&current_datetime), "%F %T");
  + sample_writer("start_datetime = " + current_datetime_msg.str());
  +
  parser.print(sample_writer);
  write_parallel_info(sample_writer);
  write_opencl_device(sample_writer);
  