message: Update REDUCE_STRUCT.hpp
  update code comments
diff: ///
  /// REDUCE_STRUCT kernel reference implementation:
  ///
  -/// Real_type xsum = 0.0;
  -/// Real_type xmin = 0.0;
  -/// Real_type xmax = 0.0;
  +/// Real_type xsum; Real_type ysum;
  +/// Real_type xmin; Real_type ymin;
  +/// Real_type xmax; Real_type ymax;
  +
  ///
  /// for (Index_type i = ibegin; i < iend; ++i ) {
  /// xsum += x[i] ; ysum += y[i] ;
  