message: print warnings only in debug builds
diff: // Error handling and debugging
  // ----------------------------------------------------------------------------
  +#ifdef NDEBUG
  + #define OIDN_WARNING(message)
  +#else
  #define OIDN_WARNING(message) { std::cerr << "Warning: " << message << std::endl << std::flush; }
  +#endif
  +
  #define OIDN_FATAL(message) throw std::runtime_error(message);
  // ----------------------------------------------------------------------------
  