message: Made Visual Studio work around only for windows
diff: namespace hmdf
  {
  +#if defined(__linux__) || defined(__unix__) || defined(__APPLE__)
  +template<typename T, typename V>
  +std::unordered_map<const HeteroVector *, V> HeteroVector::items_;
  +#endif // defined(__linux__) || defined(__unix__) || defined(__APPLE__)
  +
  +#ifdef _WIN32
  template<typename T, typename V>
  std::unordered_map<const HeteroVector *, V> HeteroVector::items_{};
  +#endif // _WIN32
  // ----------------------------------------------------------------------------
  